This is a post about some useful GNU/Linux programs I've recently discovered. I use CentOS, and RPMs for these packages are available from karan and/or DAG.
I bought a Sandisk Sansa MP3 player in late 2005. I don't know how I got through the workday before I did that. I've bought two more since then (a larger storage capacity each time). Sansas basically work like external USB hard drives, making them Linux-friendly: you can just drag-and-drop MP3 files onto them. The Sansa's firmware then reads the files' ID3 tags to display a list of available music (ID3 tags are bits of data in an MP3 file which give the artist name, album name, track title, etc.).
Sansas are not compatible with iTunes, and I haven't tried any of the other online music services--I just rip my own CDs to MP3 files. I use grip to rip the CDs. grip is basically a nice, feature-rich graphical interface to cdparanoia and LAME. It'll connect to a CDDB site (like freedb.org) to download album and artist names and track titles, rip the CD tracks to WAV files, then encode the WAV files as MP3s.
Although I can then just plug in my Sansa and start moving files around, it's nicer to have something to keep my music more organized. I use gtkpod for this. I keep all my music files on my PC, and then periodically change what I've got on the Sansa (the Sansa is 4GB, not large enough to hold my entire library). gtkpod is a nice program for displaying what's on my PC, what's on my Sansa, and changing out files on the MP3 player.
Although grip is pretty good about setting the ID3 tags on the MP3 files, it's not foolproof. The ID3 tags will occasionally have errors or be missing altogether. gtkpod has a feature for changing ID3 tags, but I haven't had much luck with this--it sometimes even causes gtkpod to crash. So I usually just use the command-line utilities in the id3lib package. id3info lists a file's ID3 tags, and id3tag and id3cp can be used to change them.
The last software package I want to mention has nothing to do with music. It's called grisbi, and it's a pretty good personal finance program. Although I've never tried Quicken or Microsoft Money, grisbi is probably pretty comparable. I use it to track my checking account. grisbi lets me define a list of transaction categories, and I can tag a transaction when I enter it. grisbi keeps up with my account balance and has features for bank statement reconciliation. It can also run reports, handle scheduled transactions (for things like automated drafts and deposits), and track multiple accounts. I've found it to be a convenient way of balancing my checkbook (much less error-prone than scribbling in the check register).