I have recently found the world's largest online chess database--The Fics Online Database-- at http://www.ficsgames.org/stats.html which has more than 200 million games in it! If you need games to feed your SCID this is it!
Other good free online databases I have found are: 365chess.com and www.chessbites.com.
I use online databases to play solitaire chess and when I'm reading a book and they give a game example I look up that example on one of the databases and play it out on my winboard. Also I use chessgames.com when I am looking for games that someone found interesting in some way rather than just blindly picking a game from a database. Also chessgames.com have the games from some of the famous chessbooks already collected together.
If anyone else uses a different chess database let us know! Also tell us how you use databases to improve your chess.