Google Groups
Amazing 20-year archive of Usenet postings, allows you to search for discussions and post messages to these groups.

Yahoo Groups
Join one of thousands of email groups and engage in these discussions.

Topica
Another extensive collection of email groups (also known as lists) that you can join.

Flyertalk
Savvy group of business travelers with extensive postings on frequent flier topics and beyond.

Lonely Planet's Thorn Tree
Wide-ranging and informative discussions on destinations and topics ranging from "Travelling Companions" to "On Your Bike."

Virtual Tourist
More than 200,000 members write about the places they're visited.

IgoUgo
Trip journals and images from thousands of travelers.