If you don’t know what those are, then you’re probably not interested in them.
They are plugins designed to extend your DAW (Digital Audio Workstation) by adding effects (VST) and instruments (VSTi).
The convoluted link above takes you to a search of free plugins, and they have TONS of ’em. Really good ones too. They also link to commercial plugins, so, no matter your budget, they have something for you.
They are the best site I found for free VST/VSTi and I highly recommend them 🙂 their interface is fairly easy (at least for me), and they have a great selection. They don’t host the plugins themselves, but are a sort of a directory.
They also have cool articles and other stuff on their site.
Check ’em out!