I bought a 24" HP monitor on Sunday afternoon -- on sale at Best Buy. I originally was going to get the 22" version, but after comparing the two, I couldn't resist getting the larger size and better resolution (1920 x 1200 vs. 1680 x 1050).
I'm using my laptop's screen as a second display. Everything I've read about the productivity gains of dual monitors certainly seems to be true. I'm also got my AlphaSmart Neo hooked-up as an external keyboard and using a wireless Logitech VX Nano mouse. Combined with the free Switcher software (which simulates Apple's Expose), it essentially gives me a poor man's iMac. Eventually, I hope to add a wireless keyboard of some sort and a quality laptop stand to complete the set-up.
I also bought a USB-TV tuner so I could use this to replace my TV and free up some valuable space in my room. The downside is that watching TV taxes taxes my CPU when computing and watching TV at the same time. The upside is that I can resize the TV window to fullscreen or into a small window next to whatever I'm working on. I can also pause live TV and (if I can get it working) schedule recordings of TV shows. For watching DVD's or analog stations, this set-up is almost as good as having a dedicated 24" flatscreen TV.
I'm still putting this through its paces, but absolutely love it so far.