SQL Server Concise List Of Trace Flags
http://antapex.org/traceflags_sqlserver.txt

Although this isn't complete, it is the biggest single list I know of.

This MSDN article lists the Query Optimizer Trace Flags necessary to support several changes to the QO in various QFEs and hotfixes, etc.

http://support.microsoft.com/kb/974006

Technet also has a list of various trace flags (probably already documented in the links above:

http://technet.microsoft.com/en-us/library/ms188396.aspx

This documents trace flag 610:  http://msdn.microsoft.com/en-us/library/dd425070%28v=SQL.100%29.aspx (controls minimally logged inserts into indexed tables)

Technet has another huge list of trace flags at: http://social.technet.microsoft.com/wiki/contents/articles/13105.trace-flags-in-sql-server.aspx