Eventually, using Ogg Vorbis files will be no problem - you'll be able to play them in any modern application or device. (It seems Linux users have already reached this future, at least as far as the applications go.) But until this dream becomes 100% reality, you'll need to make sure the music player you're using has Vorbis-playback capability. (It is possible to convert an Ogg Vorbis file to an MP3 file; however, I recommend against this because both Vorbis and MP3 use lossy compression, and converting anything from one lossy format to another will cause quality loss.)
This isn't as painful as it may sound; you should find that a new player will handle your audio library just as well as your old one did.
For audio players that you can run on your PC, I have several recommendations: XMPlay (freeware, Windows only) has minimal library functionality (among other things, there are no smart playlists) but it's fantastic with simple playlists and individual files. Songbird (open source, all major OSes) has fantastic Internet integration and can be extended to include more functionality. Amarok (open source, Linux with Windows/Mac versions in progress) is the best all-around player I know, but as of this writing it doesn't work well outside Linux yet.
For portable audio players that you can take anywhere, here's a big list of gadgets that should work. Personally, I use a CycloDS Evolution to turn my Nintendo DS into a portable media player; however, this does not yet work with the DSi.
You can add Ogg Vorbis to iTunes by installing Xiph QuickTime Components, and to Windows Media Player by installing DirectShow Filters. If you're using some other Windows application to play music, try installing DirectShow Filters; it just might do the trick.
Vorbis support can be hacked onto many older iPod models, as well as some other audio devices, by installing Rockbox.
Want to mess with the source behind my Open Source Tunes? They're in eXtended Module format, also called FastTracker 2 format. Check if your music editor can import them - if it can, it should be able to edit every note, every instrument, etc. If you need a music editor that can handle XMs, I personally use MilkyTracker, although you may find learning it to be difficult.
If you want to publish anything you made using a source file of one of my tunes, make sure that you do everything required by the Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Respect artists' copyright, please.
Fish songs are plain text files written in a special musical language. This music format was created for a PC game called Insaniquarium Deluxe. For more information on fish songs, including information on how to play them even if you don't have Insaniquarium, see the Fish Songs article on Insaniquarium Guide.