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.