I discovered this site a while ago: https://jacksflightclub.com/
Sign up and you'll get sent cheap flights leaving from the UK and northern Europe (I assume they plan on expanding to the rest of Europe, the Americas and so on). The only caveat is that they'll typically be leaving within a few weeks at most, quite often just a few days so it's really only useful to guys that have the job, lifestyle etc that allows them to just drop everything for a while and fly around the world within a few days time. Still, it's a useful resource.
Re: Cheap, last minute flights
I've heard of Jack's. I also use the paid version of Scott's Cheap Flights and the unpaid version of Ari's Fly Almost Free. Both can be used to monitor certain routes, although they're about as fast as me just looking up the flights on Google Flights.