You can use the Profiler GUI to playback a trace against a server. But, it's single threaded, so you're not going to be able to scale it up much. SQL Server 2012 includes a new tool, Distributed Replay. You can run that against a 2008 database. It allows you add lots and lots more machines to run a trace playback. You can really stress the system to the max that way.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning