RollerCoaster Tycoon Series

The Evil One
Offers parks, tracks, patches, and utility downloads. Also has a chatroom and a forum.
http://theevilone.tycoonplanet.com/

RCT Grotto
Offers a message board, real park recreations, and trainers. [Free membership required for some content]
http://rctgrotto.coastergrotto.com

RCT Inc.
Features parks, tracks, fan-created scenarios, utilities, community park projects, interviews, and a picture gallery.
http://rctinc.tycoonplanet.com/index.shtml

RCT2 Ride Exchange
Publisher's site includes a ride exchange.
http://www.atari.com/exchange/rct2/

RCT2MG
Features parks, tracks, and a message board. Also has a guide to building coaster inversions.
http://rctmasterguide.tycoonplanet.com/

RollerCoaster Tycoon 3 Resource Centre
Offers news, screenshots, downloads, cheats, and a discussion forum.
http://www.rct3rc.com/

RollerCoaster Tycoon Tech Info Depot
Offers utilities, save games, and technical information on the game's file formats for developers.
http://www.strategyplanet.com/rctuk/tid/

Thempark's Rollercoaster Tycoon
Offers tracks, parks, trainers, contests, screenshots, forums, tips, cheats, and links.
http://www.geocities.com/thempark101/home.html

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.
http://www.tycoonplanet.com/page.asp?page=home&home=news