photobucket is most common I think. Very easy to link the pictures (they even provide the VB code, just copy/paste) and they recently fixed their upload procedure so you can easily upload many pics at once. My only complaint is the overall interface, which is a little busy and can be hard to figure out where the options are.
I also have a free Picasa account since I'm addicted to Google. It's wonderful for hosting full albums, but not as great for linking individual pictures. No ads and very clean interface. They also provide the Picasa app for organizing all your photos and easy upload to the site.