https://scottscheapflights.com/
There is a free option, and a pay option (I believe $39 a year) and this service will send you emails on cheap flights as they are found. e.g., I've seen RT tickets from DFW / IAH to Amsterdam for $300. A couple of months back they had RT tickets from several US locations to Wellington, New Zealand for $600.
I was able to score 4 RT tickets from Denver to Tokyo for a trip this past summer...right around $2000 on Air Canada, with only one layover in Vancouver.
Just thought I'd share in the savings
