Some of my own travelogues have a page with links that are relevant for specific locations - for instance other trip reports on the Internet. The links on this page are of a more general nature.
Guides & Advice
Lonely Planet. Travel tips, maps and descriptions of places all over the world.
Fodor's Personal Trip Planner. Lots of destination guides, reviews of hotels and restaurants etc.
Will Go To. World travel directory and guide.
Travel-Library.com. Travelogues and tourist information.
Gard Karlsen's travelogues and pictures from trips to many countries.
Elisabeth & Teije's travel website
Slow Travel Trip Reports. Lots and lots of trip reports.
TheTravelzine.com. Don and Linda Freedman's travelogus.
Hotel & Restaurant Guides
Tripadvisor. Hotel reviews - check before you make a reservation!
here.com. A good map service.
Google Maps. And the Google of course...
RailEurope. Interactive maps with railways in Europe.
Fora - Messageboards
Slow travel talk. Dedicated to "Slow Travel", which can best be described as the opposite of rushing from one destination to another. Everyone can visit and read, but you must a membr to post messages.
Tripadvisor has many fora for specific locations/destinations.
TheTravelzine. There are many here who want to see everything in a weekend, but you can also find useful information and people are very helpful.