I just found this one out there and wanted to share it. It connects to an instance and show you a bunch of figures. Nothing you can't extract yourself with SQL queries, but sometimes it is just nice to have one tool which is very easy to use. Here's what it looks like when connecting to an instance with no load on it:
As you can see, there are some hyperlinked pages as well, and there are also some interesting options (like logging to CSV or for PAL analysis) under the "Option" button. One more thing I liked about it is that there is no installation, just an exe file to download and run. Here is where you find it: http://sqlmonitor.codeplex.com/.