Control templating in Avalon is super powerful, but can be tough to get right. There is a lot of minutae to keep track of to build a control template that works correctly. This is especially true for some of the more complex controls (like Menu or ComboBox).
In an attempt, then, to give you a better starting point, I've put together a handful of "simple" styles. Simple for two reasons: first is a simple design aesthetic--these are designed with the idea that they are a base for your own design kung-fu. They are also simple in implementatio--I've minimized the unnecessary goo so it should be easier to see what's going on.
You can get them here. Right now, I've only tackled the most common controls. More will be coming though. Chime in if you have requests.
Simple Styles for WPF are no longer available as a standalone download. That sounds bad and I'm sorry if it's inconvenient. You can, however, get them as a part of Kaxaml or included in every single version of Blend. So yay! They didn't gp way, the just got famous.
If you're looking for Simple Styles for Silverlight then you need look no further than this other blog post.