Garmin Edge 500. It's small, light, and works with ANT+ devices.
For a full review see Ray Maker's blog: http://www.dcrainmaker.com/2009/11/garmin-edge-500-in-depth-review.html
I've had mine since March, and I've recently coupled it with a Powertap PRO+ power meter. The combination is a bit pricey though, but there are two great things that you get from this combo:
1) Once you've figured out your power levels, you can ride right at the edge without blowing up. This is especially important on hills where your initial effort at the bottom winds up being way too much for the conditions.
2) It's great for measuring your progress over time, especially if you test yourself on the same course over a year.
The Garmin connect software is OK. What is cool is you can look at other people's numbers to see how you stack up. For example, you can check out Ryder Hesjedal's "easy mountain day" on Stage 14 in this year's Tour de France. Newbie here - I can't add the link, but you can find it by searching for "ryder hesjedal stage 14 garmin connect".