For the last year we have been using Teleriks AJAX Components. In my personal option I think that the AJAX components Telerik offers takes the UI of .Net web development to a new level. Keeping the consistent look, feel and functionality making development time a lot easier and quicker.
There leading AJAX data grid to the HTML editor they help create a SEO-friendly and high-performance Enterprise web applications. We have used a number of these components a newly released governmental based program These tools are very useful in custom web development solutions.
The AJAX Components includes 62 controls, documentation, live demos and 12 skins along with Visual Studio samples.
They also have a new Silverlight component set, which we are going to purchase and start to use. Check back soon to see what we think about it.