Since this thread was bumped by avermat thought this would be relevant. I've compiled a list of thumbnail providers here: http://sbd.cm/h55r1A. It has a list of free and paid features for each.
Thanks Snowblind1! Didn't know about a few of those. FYI - the last link on your page for "screenthumbs" goes to the wrong site.
Its even more easier and free now just try google and search for " Free Thumbnail Api " you will get plenty of sites giving you thumbnail for free and just integrate it
When I do that search a few thumbnail providers come up, but most of the results are garbage. The providers that do come up aren't the ones I would consider the best. IMO that's one flaw of Google though.
The thing about using Google to find it for you is that you don't know what you are getting in terms of reliability. You will end up trying half a dozen before you settle on one that has stability. Open Thumbshots and SnapCasa for example, are two very popular services, but during 2010 their reliability/stability has diminished significantly. They are slow at refreshing or adding new thumbshots, and quite often your directory/website is left looking awful because they have a "compiling screenshot" image in place of the real image for months-on-end. Snowblind1's round up is good because he has obviously done the research. I know this because I too have used most of the thumbshot providers on his list and Shrink The Web is by far the current best in terms of stability, refresh rates, and reliability. The owners/operators contact you via email whenever they are doing upgrades or when they expect downtime, and if you contact them you get a fast response. I'm not affiliated with any thumbnail providers in any way, I just believe in giving kudos and credit where it is due, instead of relying on Google's mish-mash method of finding things.