Is there some reasonably cross platform way to create a thumbnail image given a URL? I know there are thumbnail web services that will do this, but I want a piece of softwar
There are a number of commercial packages that will do what you want. I'm not sure from reading your question if free is a requirement. But here are some applications I've found that are reasonably priced and which do exactly what you want. I have not used them myself, but they have free trial downloads so you can evaluate before you purchase.
I know you said you want the service to be local, but... if you have to be connected to the Internet to take the screenshot, you should equally have access to a web service. It seems like a better move to do this than to open yourself up to cross-platform issues of taking screenshots locally.
You can use Firefox or XULRunner with some fairly simple XUL to create thumbnails as PNG dataURLs (that you could then write to file if needed). Robert O'Callahan has some excellent information on it here:
http://weblogs.mozillazine.org/roc/archives/2005/05/rendering_web_p.html