There are many forums that discuss explosives, with four "main" ones. They vary significantly in member quality and topics discussed. Here I give my own personal opinion of these four forums.
The Explosives and Weapons Forum
Often referred to as The E&W Forum or simply just "The Forum", this is the best place on the net for discussion of explosives. Other topics discussed here include improvised weapons and chemistry. Lurk a while before you post to learn how things work. The moderators are VERY strict. Do not post a topic (especially not a question) for your first post. I'd say wait until you have at least 50 decent replies to other threads before starting a thread of your own, but it depends on what the thread is about. Before you ask a question, scan over topics currently being discussed, search the archived threads, and search elsewhere also. Asking questions that have been discussed many times before and are easily answered will most likely get you HED, aka Hot Electron Death, meaning you are banned and your posts erased.
A relatively new site. This site is dedicated to chemistry of all kinds, including explosive chemistry. The forums are small, but growing. There is a lot of serious chemistry discussion here.
Amateur Pyrotechnics & Chemistry Forum
Broke off from Xinventions when that place went to hell. Run by MrYuk, I am also an admin here.
Temple of the Screaming Electron (TOTSE)
There is a lot of stuff on TOTSE, most of it of "questionable" quality. Don't bother reading the explosives files in the "ka-fucking-boom" section, most of it is garbage. Check my Links page for sites with much more reliable info. Most of the members here are beginners, but there are some people with a lot of experience as well. I used to be a mod here but left to focus on APC (above link). Many members of TOTSE came with me to APC.
Once a kewl paradise for garbage and idiots, Bombshock has gotten significantly better, believe it or not. There are now some decent members that know what they are talking about, something that couldnÔø‡t be said not too long ago. Still the weakest in my opinion, but if you are a beginner you might find it useful.
Check the alt.engr.explosives and rec.pyrotechnics newsgroups as well, there are some good people and the Google archives have tons of stuff.
If you know any (major) forums I missed, email me.