+2  A: 

I guess when you talk about "UIs done in WinForms" you mean UI or widgets framework?

Free:

OpenTheme: A free basic theme library.

Dock Panel Suite: Similar to the panel system in Visual Studio 2003.

RCM: Another free skin library.

XPTable: An extended list view.

And you can find a lot of custom controls in a place like CodeProject.

But, if you are willing to pay you can find gorgeous UI frameworks on the market. Generally, they offer a free trial:

DevExpress

Infragistics

Syncfusion

...and many more.

Francis B.
A: 

Telerik has an exceptional set of winforms and WPF controls. It is not cheap but they may have "Free" versions.

Edited:

Actually found a free component from Telerik. I have not used this before however.

Click here and look at RadFormDecorator.

Cody C
That's for ASP.NET Ajax only, it seems.
Kawa
Love the name. 8) RadFormDecorator
Sneakyness
+1  A: 

Plastic SCM from Codice Software is a very nice example of what a non-traditional, cross-platform WinForms application could look like. It almost has a WPF look and feel to it - but is completely written using standard WinForms (and NO 3rd-party controls!). The screenshots I linked to above to could be good inspiration for you.

The application has a trial period in which you could download it an play around in the UX that they build. The developers are easy-going and would probably even be able to find some time to answer some "how did you do xxx" questions.

ckittel
Thanks, that's the kind of thing I was looking for.
ChrisC
A: 

Personally, I'm a Devexpress addict. It's serious company, exist from a long long time. They was already there when I used Delphi a long time ago and the VCL (Delphi) are still supported. The support answer, help quickly and the community is very active.

It's not free (there is a free 60 controls pack I think) but I never see a professional controls pack for .NET free, up to date, with (long time) support .....

Kris-I
+3  A: 

Also take a look at Krypton - there is a free control library.

kevinw