I've been using Snipshot for ages now as as v. quick, in-browser method for generating web ready images on-the-fly. For web purposes it's great.
But it's not ready yet for Firefox 3.0 and has thus disappeared from my right-click.
What's replaced it is Picnik, which is similar photo editor but with more options and also a firefox add-on (Facebook too). It is Flash based, takes slightly longer, it does try to sell to you and it is tied into Flickr.
But that's being picky. Recommended!
All new content on my restarted blog is here