I had an amiga, but I borrowed an atari st a couple of times and was quite impressed.
If you've got a way to write 3.5" disks, I think you could download images from the net and make some bootable disks. But I've seen adaptors for ST's and Amiga's that let you use modern flash memory as an alternative mass storage medium. Modern flash memory stores considerably more then the 350kb or 700kb you could get on a 3.5" floppy ! I seem to recall that either the amiga or the st had an unusual drive mechanism where the spin speed was proportional to the position of the head. Not sure.
Anyway did a few searches and you might find some of these useful :
LGD - For Atari ST Games, Reviews, Demos, Music, Downloads, Box Scans and Much More!
http://www.atari.st/
The Atari 16-bit computers at the Atari Museum
http://www.atarimuseum.com/computers/16 ... tarist.htm
Atari Arcade
https://www.atari.com/arcade#!/arcade/combat
Satandisk - Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Satandisk
Atari UltraSatan and SatanDisk SD Card READY TO USE - £19.99 : Atari ST Sales, Software, Hardware, Buying, Refurbishing, Spares and Selling for the Atari ST
http://www.ataristsales.com/index.php?m ... cts_id=193
Atari ST HDD - Atari-Forum
http://www.atari-forum.com/viewtopic.php?t=20782
Guide to Flash Storage for Vintage Computers - Retro Games Collector
http://www.retrogamescollector.com/flas ... computers/
If you'd like to tinker and you can get a few boot disks, then it might be possible to transfer software from your pc using the serial interface (RS232). Pretty sure the Atari ST has a serial port as a guy I knew used an ST to talk to his Terminal Node Controller (TNC) for packet radio.
Or if you can get hold of a usb floppy drive you may be able to write disks that way from your PC. Or maybe you have an old PC or laptop with a 3.5" floppy. Few searches reveal :
Make ST disks on a PC ? - Atari ST/TT/Falcon Computers - AtariAge Forums
http://atariage.com/forums/topic/181242 ... s-on-a-pc/
Writing Atari ST programs onto floppies with PC - Atari-Forum
http://www.atari-forum.com/viewtopic.php?t=11708
Floppy Image program
http://atari.8bitchip.info/floimgd.php
So looks like you need some special software to do it.
Be interesting to hear what you do with it. I think a Joggler would sit nicely on top of it and could be used as an intelligent monitor ? I think emulators are great but it is nice to go native...
HTH