You Control Desktops (YCD) does what Spaces does, but has way more options and works in Mac OS X 10.4.

I have been using YCD for more than a year, in OS X 10.4 and now in 10.5. When Leopard's feature set was reported, it struck me that the Spaces function was the answer to my cluttered desktop. (I had used Jonathan Sav's Virtual Desktop before, but was turned off by the buggy nature of the product)

YCD lets you set up several virtual desktops, or workspaces if you like, along the lines of Spaces. But it allows you to have much more control over what appears where than Spaces. Unlike Spaces, you can set certain windows to appear in every window – handy for instant messaging apps as you move from window to window. You can also collect windows to whatever desktop you want. Different desktop pictures works well for me, reminding me of which desktop I am on.

Working with YCD in 10.4, losing the functionality for a while after upgrading to 10.5 (until You upgraded the program) was a real pain. Spaces is quite hamfisted, and there are serious compatibility issues with, amongst others, Microsoft Office applications which have yet to be resolved, never mind the bizarre window behavior (random disappearances, flipping desktops...).

YCD is not for everyone. But I'm quite happy with this product. Maybe Spaces will grow up in 10.6. Until then, I'll be using You Control Desktops.