Shane's Chess Information Database is a powerful Chess Toolkit, with which one can create huge databases, run chess engines, and play casual games against the computer or online with the FICS. It was written originally by Shane Hudson, and has received strong contribution from Pascal Georges and others.
Scid vs PC began with bug fixes for the computer versus player features of Scid (hence the name), but has evolved into a solid alternative with many new features and interfaces. The project is authored by Stevenaaus and numerous contributors.
Scid vs PC (modern) is my (adch99's) fork of Scid vs PC, where I have tried to make some small changes to the interface as I was not very happy with it. All major development takes place on Stevenaaus's fork only, and you should rely on it for stuff.