I don’t know if you want to include non-OW sites. If not, just delete this message, but I use the following most frequently.
To check OW fares (may be more up-to-date than official OW sources), but particularly to check on availability, I use:
http://www.expertflyer.com/
To check on whether one should use a ONE segment or pay for a segment separately, I try to find the best separate fare on:
http://matrix.itasoftware.com/cvg/dispatch