The Deck and Fusion Ads are my two favourites. They're both aimed at a Mac/design type audience.
The Deck says: "We’re picky about the advertising we’ll accept. We won’t take an ad unless we have paid for and/or used the product or service. Sell us something relevant to our audience and we’ll sell you an ad."
Fusion says: "Our passion is to serve ads that are downright gorgeous, accepting only relevant advertisers of the highest calibre. We choose from among cream-of-the-crop publishers who exclusively show only a single ad per page. [...] In order to maintain our high standard of quality, all advertisers, products and creatives must be hand-approved and found suitable for the network. We reserve the right to reject any products or services that we do not endorse or personally believe in."
(Sorry, can only link to one of the above as I'm a new user here! EDIT: Thanks Lèse majesté for correcting this.)