Flyakite OSX (http://osx.portraitofakite.com
) is completely free and dosen't slow your computer at all. Its mostly a theme and icon pack with its own installer. It does other things like skin the log in screen and when windows is loading it replaces the graphic with a mac one, I've actually changed some of the icons on the desktop to non aqua ones. Basically it skins everything to look like a mac and looks very nice, it also skins Internet Explorer (not too well - give me a good safari firefox skin) and MSN messenger. It also has its own installer and unistaller and is supposed to be trouble free to remove, I've had it on my comp for about 6 months and have not felt the need to get rid of it yet.
My Dock (Objectdock) is a very nice little program, it has very little overhead when your not using it, certainly nothing you won't notice and it works with windows perfectly, it dosen't suddenly activate when your playing a game or something.
Konfabulator is the little dashboard program, its really quite neat but you need to pay for it (I have yet to reach the end of my trial and haven't decided if its worth it), it dosen't really have any cpu overhead and is very stable, however it does have a fairly substantial ram consumption at a couple of megs a widget. If you have a sizable amount of ram (like 1 GB) then its nothing to worry about, I would be kinda concerned about running it with all the widgets I have on 512 MB and I wouldn't even consider it on 256.