Photobucket and Tinypic, most often.
Depends on what I uploaded the image for. If it's an image for my site, I upload via FTP or attachment. Otherwise, I just use Photobucket.
Use to use ImageShack with the ImageBot FF plugin. ImageShack kinda got dodgy for me, so I generally use TinyPic.
My own site, however if I don't I use
TinyPic!
Best. Image. Site. EVA.