Doctor of Credit – this is more a site oriented at deals of all kind (not just travel), and regularly subscribing to this site’s updates could easily save you hundreds (if not thousands) of dollars a year.
The Flight Deal – the best resource for finding out about unbelievably low airfare deals
FlyerTalk – the first and ultimate forum for anything travel-related. There is a bit of a learning curve (especially of acronyms) necessary before the posts can be of use, however.
ITA Matrix – the best airfare search engine
Million Mile Secrets – a great blog for frequent flyers, and one of the more comprehensive ones out there
One Mile at a Time – hard to find a better resource out there for comprehensive reviews of airlines and hotels all over the world
Reddit r/churning – while Reddit itself needs no introduction, this subreddit is an excellent group of intrepid credit card enthusiasts which takes great pride in being the first to break great deals and offers (and for those looking for something a little funnier and less useful, I highly recommend r/shittychurning)
Skift – travel journalism site with excellent original content
Travel Codex – another excellent blog for frequent flyers
View From the Wing – one of the first, and still one of the best blogs out there for frequent flyers