I've always used
ImageShack (http://www.imageshack.us) which is free, quick and easy (no sign-up necessary). (It can host videos too, though that's a new feature and I haven't tried that yet.) It can do clickable thumbnails for forums as well as direct linkage with no hassle, except in the unusual case that an image gets too popular and bandwidth overusage kills the image.
The only other major gripe is that while I'm at work I can't access ImageShack and I can't view most images hosted there.
Anyone know of any alternatives? I'll probably stick to ImageShack when I can, but in the odd case I might really need to see or put an image on these forums while ImageShack is restricted, it might be nice to know where else I can turn.