Update 1/10/2010 : Reuxables have now released some free themes including a lightweight version of Gemini (below) that has just the two colours. I have a sample application here showing the theme in an application I have just released.
Update : As rightly pointed out by a reader, Reuxables have stopped their free offer as they are now concentrating on the new .NET 4.0 controls and themes, including the elegant Gemini to be released soon.
It seems the only freely available themes are the ones one codeplex. These were originally designed by the reuxables team, but I have lost faith in them with the withdrawal of the free themes they had available.
I think the best free themes at the moment are the Silverlight 4 themes, but they are yet to be ported to WPF.
A little while back I noted that reuxables were gathering steam. Slightly earlier that year, we were all wowed by the Lawson Mango application
as this was a clear illustration that WPF could offer a vector based visual experience that left your ‘jaw on the floor’. Reuxables have now released a free version of the above theme called Inc
I have been using this in my WPF applications, and have grown rather quite fond of it, principally because WPF themes can be a bit ‘too much’ for applications that one uses daily. You can get the theme here (there is also a free Metal theme [below] if that takes your fancy)