Transitions originated from the ashes of the old Ironman Australia forum (called OzChat). People basically gave up on the official IM site forum after it was shut down for long periods of time after an Ironman one year, and there was a lot of dicussion of whether it was a form of censorship. Transitions has a poor signal/noise ratio, and you will find lots of off-topic posts, happy birthday greetings and other junk.
Slowtwitch has a lot of interesting information on their website, though their forum can get off topic (especially on US race topics, which don't always interest people in Australia). Although I've lived and raced in North America even I struggle to stay interested in topics on their forum.
That's about it really for Triathlon.
There is also
SleepMonsters for those who are bent towards Adventure Racing.