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.