You can download Winboard with Rybka 2.2 from this very website!
Free Engine

This is obviously a very late post but only just found it. I read all the posts with great interest. I've bought two versions of Fritz but the only reason I bought them is because they are the only programmes that work with my DGT board.
I don't find the Fritz program very easy to use at all and was wondering if there was any program out there that would work with the DGT board. Is there a free engine out that would work with it? I have been told that Aquarium is more user friendly but this does not support the DGT board.

Aquarium is more user friendly but this does not support the DGT board.
For me, Aquarium was the exact opposite of being user friendly.

If you still want a free engine, Stockfish is good. It's stronger than Rybka 2, and is open source. http://www.stockfishchess.com/
I am late to this party but I have to say I have tried all the interfaces discussed here and for me (and yes this is just an opinion), the easiest one to work with is Shredder's GUI from Shredderchess.com
Arena's analysis functions were goofy and would not let me work in the way I wanted to. I had to spend to much time diddling with engine options. Same thing goes for Aquarium and it just had too many bells and whistles. Chessbase products are always dicey and convoluted and their customer service....wait...what customer service!!???
I really do highly recommend the Shredder interface. It is easy to get right to work, it supports UCI engines, and it is solid as a rock!! (Remember the guy who wrote shredder came up with UCI)
I have Shredder 11 and Hiarcs 12 installed on the Shredder GUI and am VERY happy!