Oh man, there's a lot of choices here.
I'm going to not really pick software here, but rather a language you can write software with-Java. I love programming in java, and the possibilities are endless.
Software wise though, linux regardless of the distro is nice, it pretty much has a free version of any program you could ever choose.
Now that i'm done beating around the bush, i'd say rythmbox for music, firefox for internet, and pidgin for my instant messaging needs.