Titan Quest is co-op and underrated- and you can play on one account doing the ol' login switch if you only have one copy.
Left 4 Dead 2 is now on Steam Workshop, and now getting mods is really easy. Same with Portal 2, although Portal has an easy to use level designer that you could use.
Borderlands 2 is great for co-op, though I'll issue you a warning: Becareful on the playthrough 1 of doing every sidequest and overlevelling. It makes the easy sort of lame, and the loot doesn't scale that well (if you overlevel), unless you are on the new game + (Or "Vault Hunter Pro")
Co-optimus has a comprehensive list of Co-op games - although you'll have to click on the pages to get more information like LAN or splitscreen, or story etc.
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