That would depend on the site you are logged into. With Stumble you click the thumbs up icon on your SU toolbar, then give it a review. For Digg, you have to "digg it" which is something I've never done. All of them are pretty similar, though each is slightly different (mainly in terminology). If you give which site, then we can help you better... In most (if not all) cases though you'll need to make an account on whatever network you are referring too, and go from there.
It's not necessarily about PR, but if you post on these sites with a keyword anchor text then you are also going to get higher SERPs -> more backlinks of course.
Excellent resource to share, thank you! Now to spend a few minutes cleaning it up/integrating with existing resource list and find out what I've been overlooking.