I use Tumblr, simply because I already use Tumblr to keep an eye out for new CC from some of my favorite creators and so I already have an account there and login to it all the time anyway.
As far as I know there isn't even a limit on how much you can upload there on a free account.
Imgur also works great (and doesn't even require you to have an account or login to use it for image hosting.
Another great hosting service is
postimages.org, which makes it super easy to get the bb code for posting the images here. If I wasn't already using Tumblr with an account there, it's probably what I would use.
