Here are links to sites that I've found useful or have an importantance to me.

Rollercoaster Database, one of the best sites on the internet for finding out almost any information about rollercoasters.

Screamscape is great for picking up the latest rumours about rollercoasters.

RCTMart is the kind provider of the RCT Architects club forums and also one of the best Rollercoaster Tycoon site's around.

The RCTspace network is another good Rollercoaster Tycoon site.

New Element is a Rollercoaster Tycoon site for very good parks and coasters.

RCT Architects site, the RCT club of which I'm a member.