Rule 34. You can lose most of the advertisements with properly setting your popup blocker if you like.
It is a tag-searchable database-type site. Doesn't tag activity, just characters, series or show they appear in, or artist if known. Search "Calvin_and_Hobbes" using underscores, and you'll get however many pages of thumbnails they've got on them. The site is sometimes slow: it puts a real strain on servers, being so large.