I went with bluga.net's WebThumbs service as it has the best support for the latest CSS3 modules that we're using, and a really easy to use but thorough API. The only thing significant that was missing was support for web fonts, which the author is already working on.
Browshot uses real modern web browsers, with full HTML5/CSS3/Flash support. You can try the service for free.
If you are looking for a wordpress plugin that will automate the job for you than "Simple Website Screenshot" does a fantastic job. You can specify the thumbnail size etc.
We use it on our own website (http://productivewebapps.com) if you'd like to see an example of it in use! (Disclaimer: I am the owner of this site)