Here are a few helpful websites. Travel at Random does not endorse any of the sites listed below, we just find they may contain some helpful information.
Know the rules for airport security. This applies to passengers in the US, although many other countries follow similar rules.
This site, more importantly the iOS app, has helped me find some great restaurants and spots to visit on many a trip.
FlyerTalk is a travel forum for both frequent and infreqent travelers. There are hundred of pages of helpful information and you can search by airline or hotel brand.
Having an account with ExpertFlyer is like having access to a travel agent 24/7. ExpertFlyer gives you direct access to airline booking inventory, fares, seat availabilty, upgrades, etc. A paid account is just under $100 per year. If you travel frequently it's well worth it.
While it's not perfect, SeatGuru is a great place to check if you're looking for the best seat on your next flight. This site has seatmaps for most of the major airlines.
This is an online forum for the aviation industry, but what we like is the enormous database of airplane photos. If you want to see what a specific aircraft on a specific airline looks like you'll find a photos on this site. This is a great resource if you're teetering between spending a few extra dollars for that premium economy seat or perhaps business or first.
Know before you go. If you have any questions about US Customs check out their site. I also highly recommend visiting the site of whatever country you are flying to to find out if you will need a visa to enter.