Yes, I think the best one is picnik. Lots of tools on there are free but some are premium, which means you have to pay for it. BUT, you most likely won't need to use the premium ones. :)
I am personally partial to www.runestyle.com
I am a Super Moderator there and have been there from it's beginning. We have amazing resources, an active member community, and a very welcoming feel.
...
Picnik or lunapic are great free web based (nothing to download) editing tools. If you need/want more features downloading GIMP is a great options. There are also free tools to add text blurbs, to...