Install this version of RK Launcher (most articles refer to an older version). This is your doc with stacks:
http://windowsedge.com/forums/?showtopic=2158

Install Transbar to make your Taskbar transparent and then move it to the top. I still like having the bar around. You'll make it white when you do the style install:
http://www.pcworld.com/downloads/file/fid,24157-order,1-page,1-c,alldownloads/description.html

Install Windows Search 4 (works like Mac Spotlight somewhat):
http://www.microsoft.com/windows/products/winfamily/desktopsearch/choose/windowssearch4.mspx

Patch your UXTHEME.DLL file so you can install a custom theme:
http://www.ghacks.net/2008/01/21/uxthemedll-patcher-for-windows-xp-and-vista/

Install a Leopard theme (there are many out there):
http://neodesktop.deviantart.com/art/Leopard-2-0-58534221

Get Safari for Windows:
http://www.apple.com/safari/download/

If you really want Expose, then check this out:
http://www.petefreitag.com/item/124.cfm
or
http://www.docs.kr/

Mac mouse pointer cursors:
http://www.winmatrix.com/forums/index.php?act=Attach&type=post&id=999
(put in a folder in C:WindowsCursors) I had to go through and pick each one and create a new theme. Maybe I missed something. Either way, it works.

Oh yeah, the background image. Take your pick:
http://www.flickr.com/photos/dominoes/544155098/sizes/l/

or better yet get them from InterfaceLift, the best place for wallpapers.

Spaces? Well, you can run this app which is permanently in beta or just run the XP Powertoy Virtual Desktop Manager and get almost the same functionality (at least you can assign shortcut keys but no corners). While you're on the XP Powertoy page, install some other goodies like the Atl-Tab Replacement and Open Command Window here. They're helpful.

That's about it so far. As I continue, I'll add more to this post.