There are a ton of flickr plugins, and I just can’t seem to find one that’ll support multiple group pools.
Something that will accept multiple rss feeds and display pictures from all of them would also be acceptable.
Flickr Gallery is WordPress plugin that goes beyond the basics and allows several options for searching and displaying Flickr photos. I couldn’t tell from your question, but if you want to display each group separately (i.e., a set of thumbnails from Group 1 next to a set of thumbnails from Group 2), then this will do the trick.
On the other hand, if you want to combine the thumbnails from two or more groups into a common list, it is very unlikely that you will find a plugin to do this out of the box, and the reason is this: The Flickr API does not provide for searching multiple groups simultaneously (neither the basic search nor the group-pool–specific search). The only way to make this happen is to aggregate the RSS feeds as Mike Lee described or use the API and combine the search results programmatically.
I know of no such plugin either. Other than creating your own (which I’m guessing you want to avoid), you could consider this hack:
Yahoo! Pipes takes a bit of playing around, but once you have it, it works wonderfully. Hope this helps.