Gert Drapers, whom I consider to be a super-genius of the SQL Server world and owner of the very useful website http://www.sqldev.net, wrote a useful little script a few years ago that I thought I'd share.  This is a SQL query that will show who and what is causing the log of a database, in this case tempdb, to grow:

USE tempdb
GO

SELECT Operation,
   Context,
   [Log Size] = SUM([Log Record Length])
FROM ::fn_dblog(null, null)
GROUP BY Operation, Context
GO

This dumps the active part of the log in tempdb and provides an aggregate per operation and the context it occured in. The [Object Name] and [Index Name] columns provide info on which object, [Server UID], [UID] and [SPID] can provide user context when applicable.

Cheers,

-Kevin