Actaully because of the nature of open source applications ("I can build one of those!"), giving you a one for one replacement for your OSX apps is difficult. As you can see from the other answers for this questions there are at least a couple different applications for each type of app you're looking for.
Since there are so many different variations of text editors, music players, movie makers, desktop enviroments, the easiest thing to do would be to find a set of applications that you personally like and enjoy using. If you find that you're a Gnome desktop user, we a thing for some KDE applications and the occasional fling with Vim, no one is going to stop you.
Since you spoke of Ubuntu, I would suggest that you just go through their application library and start downloading applications that sound fun, exciting or might be usefull for you. Installing and uninstalling applications is trivial with Linux. Play with the thousands of applications out there and have fun.
Good luck and hope this helps some.
Is Colloquy just an IRC client? Or more? – Sasha Chedygov – 2009-07-16T20:42:18.150
Colloquy is a really, really nicely designed irc client - nothing much more than that, just very well executed. – Chris Adams – 2009-07-16T21:27:02.067
If your main goal is to replace your apps with open source versions (as you write), you can switch to open source apps and still use a Mac. In particular of those you list Growl, Colloquy and Quicksilver actually are open source apps. Now if it's important for you that the OS is open source, that's a different issue. – Fabian Steeg – 2009-07-16T22:13:05.077
1@Fabian: I think by open source he means free (as in beer). The way I understand it, he just wants free replacements for his Mac software. @Chris: I don't know if you'll find anything as "nicely designed" for Linux. For the most part, Mac apps look much better than Linux apps. :) (Linux people, don't shoot me.) – Sasha Chedygov – 2009-07-17T06:15:01.937