I haven't really done a comparison, but think the most important thing is how intuitive it feels as you use it. Are the menu items you're looking for right there? What will you be using it for? Sharing privately with friends whom you send a password, slideshows, or publicly to post. How easy is it for you to find the proper URL to use if posting. Want to crop, resize, produce thumbnails?

Since you're talking about free sites, I'd suggest you try a few, figure out which one works best for you, then delete the others. Or keep a couple for various uses.

Ana, who uses Photobucket

I used Photobucket for a couple of years. I found it too slow and too cumbersome to move things around. It was really tedious to organize/re-organize.

Picasa is a lot faster loading and for organizing, adding captions, etc. I think I switched over in late 2006 or early 2007.

Cameron