I've decided my OS needed refreshing and im at a loss trying to come up with programs that I'm going to need somewhere down the road.
Flux is great for your eyes. You have to give it a day or two to get used to, but then you won´t even notice it. I also use GeekTool to personalize my desktop. smcFanControl is good, I never like my computer hot so I use this program.
Reddit's most-saved comment ever is an organized list of essential freeware. This list is different (and imo better) than the list linked in giml's comment. http://www.reddit.com/r/AskReddit/comments/14drdr/what_is_so...
Obviously you'll need a browser; Chrome is great and there are loads of extensions, some of which as a long-time Chromium user I am only just discovering. Lifehacker recently made a great list (which can be found here) of Chrome tips and extensions. I recently discovered Vimium which is a great extension which allows you to control Chrome from the keyboard alone. I would recommend VLC for almost all media playback, as many others have. Flux is good too, to reduce eye-strain during computer-use when it is darker. An office suite is also good; LibreOffice for free and open-source, Microsoft Office if you need it, or even Google Drive (free web-based app) for most of your document editing needs. That's most of the generic stuff; there will probably be other programs suited to more specific needs of yours, but nowadays I do most of my stuff from the browser anyway.
I much prefer using CCCP along with Media Player Classic for my videos, for music, I tend towards winamp, though Foobar is well loved amongst my friends.
I don't really like the usability of either Chrome or VLC. I rather stick to Firefox and CCCP. Also seconding 7zip, and adding Audacity if you do any audio editing/recording, just to play around with effects or even to convert between audio formats.
I'll second Chrome and VLC. 7zip is great for compression/decompression. DC is a nice client program, Peerblock is useful. Rainmeter is really cool if you're into desktop customization. If you play music (guitar, bass, or drums specifically) then tux guitar is great for opening power tabs and guitar pro files.