Look but don't book on third-party sites. As Tom B said kayak is particularly good for searching Multi cities and open jaws. But again don't book. (they are owned by Priceline)

If there is a problem with the reservations booked by third parties the Airlines or Hotels will always defer to the third-party. Much better to deal direct. Also, the days of roundtrips being less expensive than two one ways no longer apply universally as it used to.

Use third-party sites to search, but book direct. IMHO