This site is run by Andy Lapping and is Rhapsody-specific. A great source of tricks, hints, models, and other stuff
E. Dougas Jensen's Real-Time for the Real World
If you're interested in deep and useful discussions about real-time analysis, this is a great place to go.
Willert Software Tools
A great bunch of guys, especially if you use small CPU platforms with Rhapsody. They have custom frameworks for small systems, tools, training, and consulting.
Systems and embedded software development, especially with Rhapsody. They provide consulting and training from their base in Switzerland.
Tools for timeliness, performance, and schedulability modeling.
Tri-Pacific Software Inc
Tools for real-time analysis of software. The main tool I've used here is RapidRMA for rate monotonic analysis.