Windows Form Controls
DotNetBar
DotNetBar in my opinion is very overused, but contains a lot of great looking controls to make your program look nice.
http://www.componentsource.com/products ... index.html
Price: $283.22
Nevron
Nevron .NET Vision is personally one of my favorite UI suites. It contains many great UI controls and much more.
http://www.componentsource.com/products ... index.html
Price: $1,269.10
Syncfusion
Syncfusion Essential Studio is very much like DotNetBar, but has a lot more controls to choose from.
http://www.componentsource.com/products ... index.html
Price: $1,269.10
Telerik
Telerik RadControls for Windows Forms has many stylish UI controls and many other components.
http://www.componentsource.com/products ... index.html
Price: $719.10
ComponentOne
ComponentOne Studio Enterprise is a set of very useful and stylish UI controls and various other tools that are useful for multimedia needs.
http://www.componentsource.com/products ... index.html
Price: $957.00
Infragistics
Infragistics .NET Advantage Controls are truly cutting edge components and can give your program an incredible UI.
http://www.componentsource.com/products ... index.html
Price:$1,015.75
Xceed
Xceed Components are mainly known for their code operations. It can do very comprehensive actions that are very useful. It also has a small set of UI controls that have options no other suite has.
http://www.componentsource.com/products ... index.html
Price: $767.95
DXperience
This is the ultimate toolset of great looking UI controls.
http://www.componentsource.com/products ... index.html
Price: $1,195.99
SandDock
Divelements SandDock are docking panels that are very useful, user friendly, and stylish.
http://www.componentsource.com/products ... index.html
Price: $165.62
SandRibbon
Divelements SandRibbon has a small Ribbon tool set like the Office 2007 Ribbon.
http://www.componentsource.com/products ... index.html
Price $244.02
SandGrid
Divelements SandGrid contains TreeViews that are very unique.
http://www.componentsource.com/products ... index.html
Price: $342.02
Code Helpers/Optimizers
Note that having more than one of these installed may cause problems in Visual Studio.
ReSharper
The best tool you can have. It provied many error handling options and a wide array of code optimizing features.
http://www.componentsource.com/products ... index.html
Price: $195.02
CodeSMART
CodeSMART is best paired with ReSharper. It contains many code snippets to save you hours of typing.
Price: $249.00
CodeRush
Another code helper that I do not recommend. It greatly impacts the performance of Visual Studio and does more harm than good.
http://www.componentsource.com/products ... index.html
Price: $237.49
Refractor Pro! (Does not support Visual Studio Express Editions!)
Helps developers craft and sculpt their code with the speed and efficiency.
http://www.devexpress.com/Downloads/Vis ... factoring/
Price: $249.00
Well, that's all I have for you guys at the moment. Enjoy! I want to see some good looking programs come out soon!

And if you know of any more UI suites that are not listed, please post and tell me and I will update this post.