ParticleMan
This is a user control that creates particle effects in Silverlight. The current version lets you customize the speed and size of the particles as well as a beginning and end color. Stay tuned for updates like angular momentum, gravity, some fake 3D and performance improvements.
Kaxaml
Kaxaml is a lightweight XAML editor that gives you a "split view" so you can see both your XAML and your rendered content (kind of like XamlPad but without the gigabyte of SDK). It supports WPF and Silverlight.
Nerd+Art Code Snippets
This is a set of snippets for WPF and Silverlight that are intended to jumpstart common developer activities. They original intention was for these to be simple enough that even a design could use them. This is the set of snippets that we explained in our MIX08 session, but with a handful of updates. Watch the talk and see what you think.
AnimatingPanelBase
AnimatingPanelBase is substitute base class for Panel that makes it super easy to add motion to Panels in Silverlight. It works just like any other Panel. You define your layout logic in Measure and Arrange. If you can write a Panel, you can write an animated panel with AnimatingPanelBase. This version is compatible with Silverlight 2 b2. With just a little tweaking, you could probably get it up and running in WPF too. Also, check out Kevin Moore's bag-o-tricks. There is almost definitely some "borrowed" code in here.
ImageSequencer
ImageSequencer is a WPF control that allows you to playback a sequence of images like a flipbook. This implementation is really resource efficient, so it's fast! Only catch is that right now the images can't be loaded directly from the assembly. That's probably an easy fix so if someone makes it, let me know and I'll post the update.
HandDrawnShapes
HandDrawnShapes is a collection of 3 elements (HandDrawnRectangle, HandDrawnEllipse and HandDrawnBorder) which are designed use a slightly flawed geometry and look kind of like they could have been drawn by hand.
SimpleStyles
SimpleStyles are a starting point for making control templates in WPF (no Silverlight yet). I initially built these when I was at Microsoft and they became the starting point for the SimpleStyles that ship with Expression Blend, but these are optimized for handcoding. SimpleStyles are currently only available as snippets in Kaxaml or in Blend.
Ingebretsen Neato
I had a loner Tablet PC at work for a few weeks when I was at Microsoft and I came accross a utility that lets you create a font from your own handwriting. I actually really like the way mine turned out. Not sure where the "Neato" came from, but it is pretty neat to see your handwriting as a font. Feel free to use this however you want, but I'd love to hear about it if you do something cool with it.