Travel forums are listed in alphabetical order: please note that the information provided is about the travel forum section of the site alone and not the complete website.
BootnsAll: Offers a worldwide forum covering all major continents and countries. The community is active, well moderated and flourishing.
With 18 additional topics including food, meeting up and gadgets nearly everything is covered.
Cruise Critic: Solely dedicated to the Cruise side of travel the forum is broken into individual company reviews, memberships and meeting up with other cruse lovers.
The forum is a little dated looking but does offer a mass in cruise related questions and answers.
Another popular guide book publisher that's got a popular travel forum covering most of the world. It's worth keeping in mind that it's geared slightly more towards upmarket travel than budget travel.
Nice touches on the Fodors travel forum are Trip Reports and and an easy to read recent posts that's flush with ideas.
Horizons Unlimited: Specializing in overland vehicle travel. Horizons Unlimited's Hubb contains probably the most comprehensive forum in that field. It certainly airs more on the side of dedicated travel than vacation travel.
If it's border crossings, carnet de passage questions or country specific questions the Hubb is the place to be.
Rough Guides: This major guide book player rebooted it's travel forum in 2013. It's a clean fresh Q & A style now that breaks away from other traditional forum styles.
The forum categories are bit hard to find since the reboot but with some tweaks it will hopefully develop into something a little easier to participate in.
Lonely Planet Thorn Tree: Around since 1996 the Thorn Tree is the largest and most popular of all the Travel Forums. However in 2013 it took a nose dive with a new moderation system in place that's seen the community scatter.
Still, there's a bevy of information on the Thorn Tree and it remains "the" place people turn to for up to date travel information. Hopefully they'll fix up the moderation issue and bring the Thorn Tree back to it's former glory.
TravBuddy: A smaller travel forum that offers worldwide moderated categories, meet ups and casual chat.
Don't let the smaller numbers put you off. Rather than get lost in the jungle of the larger forums TravBuddy's questions do get answered quite quickly.
TravelFish: The number one South East Asia travel forum has recently opened its forums to cover most of the world's countries. Well moderated with tactical travel sense you'll find a bevy of information here.
With just about everything on South East Asia you could possibly want to know, the new country listings might just be one of the best places to ask your first travel question.
TravellersPoint: One of the premier community driven travel forums offering worldwide destinations and topics. Travellerspoint is an exceptionally well moderated friendly forum.
Questions are asked and answered from beginners to experts alike. If you are looking for a forum with an emphasis on community this is certainly one to strongly consider.
TripAdvisor: One of the larger commercial brands in travel offers worldwide Q & A style forums. There's not much community here, more reviews on places with questions asked & answered.
That said there is a wealth of information on there if you want to read peoples opinions on specific places they've been. Ranging from tourist attractions to hotels.
VirtualTourist: Worldwide travel forum in a different breakaway style. Virtual Tourist offers you to ask a question about the location you are going rather than browse through endless threads.
There's not a lot of community here, but if it's a question you just want to ask about a location you are going then this might well do the trick.