There are literally 100,000's of tunes - and they're all freely available.
Mod music comes in different formats, some of them are 'mod' files, but there're also 'xm' (nothing to do with satellite radio), 's3m', and 'it' files. These types of files consist of midi-like music notes, with built in digitized sound-samples. The software they use to make this music are programs of the "Tracker" variety like SoundTracker, ScreamTracker, ProTracker, and a few other variants. It's not like your normal music note bars, but more like sequences of notes with effects, top down, with a column for each "channel". It's not like mp3 or wav files, because those are just compressed audio streams. It is possible to convert mod files to mp3 for things like portable players, or for in the car. A Mod file, in its original form, generally takes a lot less storage space than an mp3 file, so you can fit a ton of these files on your hard drive. You'll need a special player to play (or convert) these files. For Windows users, I recommend using ModPlug. Linux users can use XMMS with the mikmod plugin (which comes with many Linux distributions built in), or else there's also a modplug plugin for XMMS, which is of better quality than mikmod. Then, head on over to the Mod Archive, and start downloading some tunes. Aside from manual downloading of individual files, you can use bittorrent to download the whole whackload, something like 16+ GB of it. You could probably play non-stop music for a month or two with all these files. Some of the tunes are weird and silly, but some of them are fantastic. I often listen to mod files while I work, which keeps me focused. You'll want to organize your own collection over time, and filter out the crap, but once you get going on it, you'll really enjoy playing your collection.