Probably Vista. I had the RC2 Beta release and I felt that I could use that as my main OS. It just had a few problems with some of the software I was running (ZoneAlarm was one) so I still use XP. But I think most, if not all of the software will be supported by Vista in it's final release so I look forward to it. Vista to me seemed fresh and was nice and convenient. I'm thinking of building a new computer to put Vista on when I get it properly, as the one I've got now I built a couple of years ago (the graphics card can't handle the Aero feature on Vista) and the CPU could be a little faster and I could do with a dual core...oh well all in the pipeline. :-)
My top software find of 2006 is probably Spyware Terminator click here there's a lot of other good free stuff I've been using for the past few years, but I think Spyware Terminator is the best one found this year.