Amarok - The music player
Martin posted back in December about switching to a tripple-boot, having XP, Vista and Linux on the same machine. I myself installed Ubuntu a while ago and am recently switching near full-time to it. One app I really love is Amarok, one of the media players in the repositories.
Amarok is just like any quality Linux app really, it integrates well into the desktop, it has a simple, clean and easy to use interface and can do almost anything you want, all this for free of course.
The main feature that sets it apart from its rivals is the native support of lyrics and Wikipedia search. When you play a song you can click a tab in the sidebar to view the Lyrics, and the next tab to view the Wiki entry of the Band. I actually found myself using the feature a lot since I play the guitar and sing, and it's easier to just load a song I like and get the lyrics instantly.
Radios are also supported of course, and there's built in support for Last.fm too, you can easily skip tracks from the panel icon, simple and effective. I also like the way you can set Amarok to display the song being played in different ways. The default stays in the center of the screen for a bit too long for my taste, but you can set it to display in the lower portion, for less time. You can even set fonts and so on, very flexible.
While looks are a bit on the simple side I like them, but of course skins are available on KDE-Look, or you can make your own if you want to. Overall I love this music player and it's a great addition to my Linux desktop. I'm loving my Ubuntu more and more every day, it's awesome that free software is better than paid ones.