1337chess
Hosts opening books developed by Michael Wagner. The free one offered is dated but might provide a glimpse into what some of the stronger opening books might look like.
Arena
A list of opening books submitted by users of the Arena chess program.
HIARCS
Provides opening books used by the HIARCS chess engine.
SedatChess
Sedat Canbaz maintains his Perfect series of opening books at this site.
Terminator
Opening books in Terminator series developed by Sunil Nair.
According to Vladimir of Chess2U's tests the best chess openings books for chess engines are
1. Doom 4a
2. Blue 4
3. The Book of Faca GL
4. Doom D3
5. Terminator Return of Game of Throne
Some on the complete list, however, are private and in limited circulation or commercial openings books.
