SGF Collections
Kogo's Joseki Dictionary - SGF containing an extensive opening database (not games).
Ang-Yue Internet Go School - Almost 4,000 pro games
ByHeart - Games from historical masters, including a complete Go Seigen archive.
Go4Go.net - Massive freely accessible database and replay feature
Go Teaching Ladder - Thousands of commented amateur games.
KGS Archive - Get the nickname of a KGS player, and you can get all their games here.
My Friday Night Files - Cho Chikun Collection
Professional Games in Smart Go Format - Games from Japanese tournaments.
U-Go (Kombilo) - zipped collections of KGS games played between strong amateurs (6d up). Tens of thousands of games.