I'd like to round up the most common places people check (forums, communities etc) into a public spreadsheet, so everyone can use them.
Let's leave the ticket search engines a side at the moment and focus on deals that are found by enthusiasts and published on the web.
Most recently I travelled Melbourne to NYC and they found if I booked an unrelated flight between Manchester and London, the overall cost of the routes I was travelling was much lower, even when paying for an extra ticket I wasn't going to use (though I'm still not entirely sure why)
I also read recently about Skiplagged (https://skiplagged.com/) which finds flights from A - B - C which are cheaper than those simply A - B. If you are only travelling A - B this will be cheaper and you just don't hop on the flight from B - C. They're getting sued by United so obviously someone doesn't like what they're doing.
As for specifically deals, I use OzBargain (http://www.ozbargain.com.au) though this is obviously Australian focused.