Windows: From the creators of Fences comes Bins, which is basically the Windows 7 equivalent of the "Stacks" feature in Mac OS X. If you want to organise your taskbar's program icons into little convenient bins, this download is for you.
Bins is very easy to use. You just install it and then drag a program icon from the taskbar (or wherever) onto another program icon on the taskbar and you'll create a bin. The bins look a lot like Mac OS X's stacks in Grid view, only much more compact. When you mouse over your new bin, it'll show you what's inside and you can access everything easily. Mousing over any of those items will activate Aero Peek, as you might expect. It's really simple and works very well. One downside is that Bins currently does not have support for jump lists, but I have it on good authority that jump list support is on its way.
Bins is currently available as a public beta. Right now you can try it out for free, but it'll cost money when it's released. A $US5 discount is being offered for pre-orders.