I use Ubuntu. It's the most popular distribution so always easy to find out how to do things. I do audio which needs an RT Kernel but this is easy to add. Best to use the standard Ubuntu then add whatever apps you want. I think that is better than going with some multimedia distribution that far fewer people are using. The latest Ubuntu is better than ever. Very cool.
Linux may be a bit of hassle to begin with but any new OS would be. It gets easier all the time. In fact easier than starting on Windows or Mac in my opinion. Once you get used to Linux you feel like tearing you hair out in frustration when forced to use Windows for any reason.
Basic apps I use are:
Ardour
ZynAddSubFX
Gimp
Inkscape
Lightzone
3D Coat
I also have Blender and Luxrender installed but haven't done much with those.
There are of course thousands of free apps.
Also, this is a fantastic app I recently started using:
http://mypaint.intilinux.com/It is very simple but very well designed. Absolutely no unnecessary UI or bloat. The main reason I love it is because of it's tremendous efficiency, meaning I can use it on my little slate PC without any major lags. As far as I am concerned, Windows and Mac apps are fast becoming a serious joke in terms of speed and efficiency.
Here is a video of someone using Mypaint, Gimp and another very cool app called Alchemy. This guy is doing the concept art for the new Blender movie:
http://www.davidrevoy.com/?article24/I recently created a load of brushes for Mypaint. I should eventually put them on the Mypaint forum. Although you can't create bitmap brushes, like in Gimp (or Photoshop), you can endlessly tweak with the procedural brush effects. A Wacom tablet or tablet PC is essential for this app.