Free SQL Server monitoring tool:  SQL Spy

Available on Windows 2003, XP, 2000, NT4, 98.  From the web site:

• Monitor your SQL Server in real-time.
• Graphically display Locks, CPU, Memory, and I/O action in a moving line graph.
• Watch who is coming and going, minutes connected, CPU, memory and locking usage per connection.
• Monitor SQL statements that are being sent to your SQL Server.
• Record SQL statements that are coming through the procedure cache. (SQL Server 2000 sp3 only)
• See when a transaction is blocked and by whom.
• View what's being cached, as well as cache statistics.
• Kill connections to your SQL Server and keep them out. Send the Kill command on a delayed timer to keep unwanted users from connecting to your SQL Server.
• Generate HTML reports of graphs, Save External Connections and Locking to either .XLS, HTML or .CSV file
• Plus a lot more...For Free!

This application is very nice.  A simple, uncluttered UI with lots of power with a few mouse clicks.  And free!  (There are some other SQL Server utilities on the site as well.)