Open Source is about options
Sourceforge.net hosts an estimated 169,281 projects as of 2008, its clear that there are many alternatives to almost every program you would need when running linux. In many cases, finding the right one for you can be like finding a needle in the open source haystack. I'm going to outline a few of my favorites that I use on my desktop and my laptop, both running ubuntu.
Amarok- Amorok is the default media player for KDE but works well on GNOME and Xfce. Amarok has some great features. Integrated Wikipedia, Lyrics Support, Music Statistics and support for multiple Mp3 players including Creative Zen and Apple Ipod.
Thunderbird-A great mail client based on the open source browser firefox. You can connect to any mail service that offers pop3 and imap including gmail. you also have the option of integrating a date book and calender.
VLC- when it comes to versitlity, VLC player is hard to beat in my book. This cross-platform video player supports everything from WMV to DivX to Quicktime files, even partially corrupted files. While some may say it is lacking in appearance, it certainly makes up for it in pure functionality.
Pidgin-Why would you restrict yourself to just one instant message service? why not connect to them all. Pidgin makes that possible, you can simultaneously chat with friends using MSN, Yahoo, MyspaceIM, IRC and more. pidgins simple but effective (add free unlike some proprietary counterparts) interface allows you to focus on whats important. Communicating with friends and family.