If I bought a game (such as Neverwinter Nights) am I allowed to play LAN with a friend or do I have to buy 2 copies?

Yea you can play NWN LAN as the game doesn't check your CD-key if you don't connect to its Gamespy service.

GOG asks that all computers involved belong to the same household. If you play on a desktop and your friend borrows your laptop, all's good. Your friend shouldn't install it on his laptop, however, without buying his own copy.

[url=]http://www.gog.com/en/support/website_help/downloads_and_games[/url] (Scroll to #9.)

Edit: post link doesn't appear to work, but navigate there and it should answer all questions.