FRAPS is very simple - you just run the program, and it sits in your system tray. Whenever you load up a games, the FPS will show. You can also benchmark your FPS as well, although I'm not sure how to do it. FRAPS also allows you to record gameplay - and take screenshots as well. Recording gameplay does stress the CPU, and can drop your frame rates quite a bit, especially if you have a single/dual core. It displays your FPS in the top or bottom corners of the screen, you can choose where it displays it.
FRAPS is shareware and costs $37 - a demo version is available for free which is all you need to check your FPS, and can record gameplay for 30 seconds with a watermark though, and can only take images in .bmp format.