SideStep bills itself as "The traveler's search engine." It is unparalleled with respect to the ease of customizing your search for airfare. As with any search engine, you'll enter your departing and arriving airports and your travel dates. The engine will then search all major airlines and offers you the option to simultaneously search other travel sites (e.g., Priceline, Hotwire, Expedia, Travelocity, etc.) to find available fares. Note: Fares for Southwest Airlines are available only on Southwest's website.
Once the search loads, SideStep's true genius shows. On the left-hand, you'll see the "SmartSort" which allows you to narrow your search based on a variety of features including:
The best part: SideStep shows you the anticipated price of each possible feature and allows you to select and layer multiple features to build your ideal flight.
One caveat: For trips with flexible dates, your best bet is still Orbitz. Orbitz will allow you to select flexible dates in three user-friendly ways (i.e., weekend trips in a specified month, bonus days for up to three days before/after your target travel dates, and flexible stays which allows you to search a trip for a set number of dates between a specified travel window).