Mobissimo is an airfare search engine worth booking.
Like SideStep, it’s a metasearch tool: you enter your search query and it goes out to various other travel sites and brings back the results. Unlike SideStep, you don’t have to download software.
On the plus side, it searches the various online services (such as Destina, Travelocity, Zuji, Opodo, Onetravel) in addition to the travel providers themselves. On the downside it doesn’t suggest alternate airports and search those automatically.
Searching last minute fares to Los Angeles from Washington National, Mobissimo outperformed SideStep and Orbitz (finding a $311 fare on Northwest with only two days advance purchase and no Saturday stay). SideStep, though, found a $207 fare on Frontier from Dulles to Los Angeles that — even when specifying Dulles as departure airport — Mobissimo didn’t find. So it’s one more tool for the arsenal, and a useful tool, but it isn’t a silver bullet.