Expedia Affiliate (formerly IAN - includes Hotels.com) is great if you want to do more than just post banners, for example, if you want to feature specific properties. They have datafeed access.
Otherwise, if you want to stick to major brands, Travelocity is on CJ, Orbitz is on Linkshare...
Travel is a really competitive area and not an easy one for affiliates unless you put some unique work into your site and/or go for a really small niche.