The main one I like so far is 7stacks
It will work on Vista and XP but it's targeted at 7. On Vista I use a double height Taskbar with the captions for running programs turned off. With Taskbar locked that leaves me with a lot of open space. I also use RocketDock. But on 7 you don't get the separation between QuickLaunch and Taskbar. The running app just glows the icon. I'm not all that thrilled when these giant previews pop up since they obscure RocketDock.
With 7stacks I can create some stacks on the Taskbar of stuff I use most. Internet stuff, editors, file handling programs, hotkey utilities, etc. to increase the free space in the middle of the taskbar. On some items it means an extra click but the glass looks cool when it pops up.
Right low it's limited to a total of 10 stacks, but it's free. I've been using it for a few weeks and I don't see any inherent bugs. The only proviso would be since you make stacks from a folder of shortcuts, you have to be cautious of creating and endless loop if you put a shortcut inside a folder and it goes to a folder with a shortcut to itself or something. I tried to use some folders from RocketDock but I think the nesting did me in since when I pressed FARR hotkey I got four explorer folders opening instead. I put it back the way it was and all was fine.
One comment though, it wants to create a shortcut on your desktop when it makes the stack. If you are like me and don't want to run with desktop icons anymore, you may want to have a folder of app shortcuts where you can hide stuff away. Just create the stack, then drag & drop the desktop shortcut to someplace out of sight To add new items to an existing stack, you just drag a new shortcut into the folder that's the source of that stack. For example if you created a stack from a folder of Internet program shortcuts and you installed yet another browser, just move the icon to that Internet folder. It will show up in the stack right away.