We've heard feedback from ISVs evaluating Silverlight that the control set just isn't enough for building the apps they'd like to build. Of course our feedback has been that things will continue to improve as we progress toward Silverlight 1.1 and that message still holds true. But for those who want to get started with a rich control set today, a surprising number of options exist.
You might want to start by checking out the Silverlight 1.1 Alpha SDK. Inside you will find the Silverlight UI Controls sample which gives you the source for several types of buttons, a ListBox, ScrollBar, ScrollViewer and Slider.
Next you could check out several Silverlight projects on CodePlex. Control kits have started appearing and a few of them show promise.
And of course, third party vendors are really starting to step up to the plate and provide control sets of their own. I was especially impressed with ComponentOne's extensive Sapphire library, and Infragistics, a great vendor of WPF controls, is also reaching out to extend Silverlight. They have some cool things on the way, especially in the realm of data visualization.
Finally, I always recommend you keep an eye on Silverlight.net. If you haven't been in the last four weeks, you might be surprised at how much great content's been added.