Thumbalizr is a new web services which enables to create screenshot of any page you want. It is free and you can choose size you desire for thumbails (150, 320, 640, 800, 1024 and 1280). In addition, you can choose between a screenshot with a screen or a complete page.
After a test, results are good but the service is a litte slow… Because of Alpha version ?
oh if the service is slow, it must be caused by the so-called blog ramenos effect