A nicely organized collection of links is found on Adam Sandy's Amusement Park History site. (Sadly, Jared Costanza's site, formerly on aol, is no longer available.)
Chris LaReau's InCoasterPaedia site is essential for historical research.
Dave Althoff's site has a number items of interest related to ride safety and technology.
The Coaster Enthusiasts of Canada site maintained by Richard Bonner contains, in addition to information on Canadian parks and rides, useful guides to books and videos. The series of articles on Closed Canadian Parks is of particular interest. The site also includes special lists of links to historical and technical & safety sites.
There are numerous sites with photographs of roller coasters, many of them very good. One of the first is Joe Schwartz's Joyrides site. It hasn't been updated much, recently, but is still one of the best. Others of note include Joel Rogers' CoasterGallery and Carl Ragan's Ultimate Coaster. I have linked to pages on each of these sites to illustrate specific rides.
The most comprehensive on-line list of roller coasters is Duane Marden's Roller Coaster Database. This is an excellent resource for operating roller coasters worldwide, complete with links to official park websites and pictures of many of the listed coasters. An increasing number of historical coasters are also included.
Michael Pantenburg's site on the rides of Anton Schwarzkopf is packed with ride information, photos, and technical data. This is a model for what a ride web site should be.
The most extensive interactive online source of roller coaster information (and, sometimes, misinformation) is rec.roller-coaster. A news server and a newsreader client are required. Although the number of messages has decreased since the peak in 1999, reports of USENET's impending demise appear to be greatly exaggerated.