Power Without The Price
In the mid-1980s, several of the home computer manufacturers moved from the 8-bit MOS 6502 CPU to the Motorola 68000 which featured a 16/32-bit architecture (a 16-bit data bus and 32-bit registers and instruction set). In 1984, Apple introduced the 128KB Macintosh for $2,495. In 1985, Commodore brought out the Amiga 1000 with 256KB of RAM. In that same year, Atari launched the Atari 520ST with 512KB for $799 (with monochrome monitor; $999 for color). Atari would later bring out the 1040ST with 1MB for $999 with a monochrome monitor, making it the first computer to break the $1000/MB price barrier.
At launch, the Amiga had a lot of technical advantages: the Amiga had a multitasking operating system. It also had chips dedicated to graphics that allowed hardware scrolling, sprites, fast memory moves (blitter operations), and more onscreen colors than the ST. It was not until the release of the Atari STE in 1989 that Atari was able to close this technical gap. But Atari would not get a multitasking operating system until four years later in 1993. The Atari vs Amiga debate was a heated one, and it still goes on today. Atari's catchphrase was "Power without the price," and if price was your driving concern, there was no debate.
However, the Atari ST had other advantages. The ST had built-in MIDI in and out ports, and there was a lot of software that could work with MIDI hardware, making it hugely popular with musicians. Using software called a MIDI sequencer, you could lay down tracks on a MIDI keyboard or guitar connected to the ST's MIDI-in port, edit them in the sequencer, and assign them to whatever instrument you wanted and play it back on a MIDI synthesizer connected to the ST's MIDI out port. By successively laying down tracks, a single musician could compose complex multi-instrument arrangements.
Software such as Cubase, Band-in-a-Box, Logic Pro (previously Notator and Creator) all have their roots in the Atari ST. Modern computers can be easily outfitted with USB MIDI capabilities, but the Atari's built-in MIDI hardware made it a first choice for sequencers and other MIDI tools. Tim's Atari MIDI World includes a list of applications such as sequencers and algorithmic applications, many of which have been released as freeware.
To this day, some music studios keep an Atari ST or two around because they still can hold their own against modern computers. The fact that the ST was single-tasking meant that your MIDI sequencer software could be in complete control of the CPU, and the fact that the MIDI hardware was built into the computer meant little to no latency.
The ST was a strong business computer as well. It was a popular desktop publishing platform, driven in part by Atari's inexpensive laser printer. It had no built-in processor, relying entirely on the Atari ST to render pages and send them to the printer. It shipped with a bundle that included the Mega ST configured with 4MB, a 30MB hard drive for $3,995, less than you'd page just for a comparable laser printer alone. In addition to desktop publishing, the Atari ST software ecosystem included CAD, spreadsheets, word processors, databases, and graphic design software.
Because Atari had a stronger marketing focus in Europe, it had a much stronger community there. As a result, much of the great software came from that continent. The leading desktop publishing software for the ST was Calamus, from a German software company named Ditek. It was later distributed for the US market by ISD Marketing. As you prowl through shareware, freeware, demos, and abandonware for the ST, you'll sometimes come across applications and games that were never translated for the US market.
The ST could be turned into a Macintosh with the help of a hardware/software product called the Spectre GCR. You needed to procure physical Macintosh ROM chips and install them into the device, but at $300, and combined with the low price of the Atari ST, this was a cheap way to get Macintosh compatibility.
Games and Demos
The ST was also an amazing games platform, despite the lack of hardware sprites, scrolling, blitter memory operations, or a full color palette. When the STE models were released to address these limitations, the ST gaming market was mature, so there wasn't tremendous incentive for game makers to rework their games to use the STE's features. However, you can find copies of older games that have been enhanced to take advantage of STE features. So, even if you have the original floppies for some of these games, if you have an STE, it's worth your time to try out these versions.
The Atari ST was popular with demoscene coders, teams of programmers who loved to push hardware to its limits. They would develop and release demos that combined graphics and music, and it was in this scene that some of the Amiga vs Atari rivalry would play out. Despite the Amiga and STE's advanced capabilities, Atari ST demoscene artists would show off their skills by creating demos that seemed impossible given the limits of the original ST models. Demosceners are still creating Atari ST/STE demos to this day.
The Atari ST Today
The Atari ST family is still very much alive today. There are many emulators that allow you to experience it, but if you have a working Atari ST, there are many upgrades/mods that you might find useful:
Hard drive emulators:
ACSI2SD is an affordable kit that allows you to use an SD card as an Atari hard drive.
UltraSATAN also turns SD cards into Atari ST hard drives.
Regardless of which hard drive kit you use, you'll need software to prepare an SD card to be used as a hard drive. These utilities include a disk partition program and a driver program. PP's hard disk driver costs 10-15 Euro, and works very well. ICD Pro is free and is also a good choice. HDDRIVER includes a lot more functionality and can support exotic configurations, but it costs more (46 Euro).
If you want a small, cheap, portable MIDI keyboard, look no further than the Rock Band keyboard. You'll find versions for Playstation and Xbox, but the Wii versions are generally the cheapest (about $25 at the time of this writing). It has a 5-pin MIDI out connector, and you can plug it directly into your Atari ST and use it with a MIDI sequencer (Cubase Lite or Sweet 16 are good for beginners).
If a guitar is more your style, the You Rock Guitar Gen2 is an enjoyable MIDI guitar. It can be hard to find on eBay (there are lots of Gen1s available, but the Gen2 is better overall). Although the You Rock Guitar does not use real strings, the fretboard feels a lot like strings. The Rock Band Fender Mustang guitar is a more affordable MIDI guitar, but it uses buttons instead of the string-like sensors on the You Rock Guitar. At the time of this writing, the Wii version was going for about $65.
You will also want some kind of MIDI synthesizer so you can play back your MIDI tracks. Virtually any MIDI synthesizer with a 5-pin MIDI in will work, but you can also build your own with a Raspberry Pi. You can order a PCB for the ClumsyMIDI Raspberry Pi hat from PCBWAY, and order the parts to populate it and build it easily (all the components are thru-hole; see the ClumsyMIDI README for complete details). With the mt32-pi software, you can either emulate a classic Roland MT-32 synth (requires you to find dumps of the original ROMs) or use any of the freely available MIDI Soundfonts which sound fantastic.
What's more, with a MIDI synthesizer, you can play some of the many Atari ST games that include MIDI out for their soundtrack, such as Ultima IV. The polyphonic synthesized music is way beyond what you get from the computer's native audio output.
Floppy Drive Emulator
The widely-available Gotek floppy drive replacement uses a USB memory stick to load floppy drive images. It will not handle copy-protected images, so you will either need to look for versions of your floppy disk games that have had the protection removed, or keep an old floppy drive around for when you need it.
The Computer Museum's Atari ST includes the mods listed above. Come check it out!