New niche online travel sites are chipping away at the market share of Orbitz and Expedia - and other online travel agencies (OTAs), according to eMarketer.

Niche sites Farecast and FareCompare, for example, help consumers determine the best time to book airline tickets. Travel social networking sites like Gusto.com let visitors share itineraries and trip info, and travelervideos.com lets travelers upload and share vacation photos and videos and write their own travel blogs.

Travel supplier sites are also edging in on OTA customers. And travel search engines such as Kayak and SideStep, as they gain traction with travelers, will drive more visitors to supplier sites. Then there are travel portals like Yahoo Travel that offer a full suite of travel products and services.

Travel is that rare category in which purchases are influenced more by websites (33 percent) than word-of-mouth (9 percent).