Atari Forums: RollerCoaster Tycoon Games: Publisher's discussion forum. [Registration required for some features.]
RCT Grotto: Offers a message board, real park recreations, and trainers. [Free membership required for some content]
RCT Inc.: Features parks, tracks, fan-created scenarios, utilities, community park projects, interviews, and a picture gallery.
RCT2 Ride Exchange: Publisher's site includes a ride exchange.
RCT2MG: Features parks, tracks, and a message board. Also has a guide to building coaster inversions.
RollerCoaster Tycoon 3 Resource Centre: Offers news, screenshots, downloads, cheats, and a discussion forum.
RollerCoaster Tycoon Tech Info Depot: Offers utilities, save games, and technical information on the game's file formats for developers.
RollerCoaster Tycoon UK: Features reviews, competitions, tips, and patch and trainer downloads. Also has a discussion board for members.
The Evil One: Offers parks, tracks, patches, and utility downloads. Also has a chatroom and a forum.
Thempark's Rollercoaster Tycoon: Offers tracks, parks, trainers, contests, screenshots, forums, tips, cheats, and links.
TycoonPlanet: Offers downloads, tracks, patches, a hall of fame, game secrets and cheats, reviews, and a list of how to abuse part guests. Also has a chat and forum.