To keep track of the ballooning list of frameworks and helpers that are required or helpful when working with Silverlight and/or WPF, I've started a list (in no particular order) here:
1 Silverlight Tools for VS 2008 SP1
2 Composite WPF and Silverlight (Prism)
3 Silverlight Toolkit
4 RIA Services
5 Silverlight Unit Test Framework
6 This Microsoft hotfix for a problem with unit tests in projects contained in the same solution as a Silverlight project.
Useful control toolkits:
1. Dev-express Silverlight DataGrid
2. Visifire open source data visualisation tools (charting)
Hopefully VS 2010 will roll some of these up and they won't have to be downloaded separately.