Is there an equivalent to xkill for Windows 7?

Process Explorer can do this just fine. You use the icon in the toolbar on the top which looks like a target icon (It's next to the binoculars). As soon as you press it, your cursor turns into that target icon, and the process explorer window is hidden. Drag it over the window you want nuked, and release the mouse button. The process who created that window will be hilighted inside the Process Explorer UI. You can nuke the process at that point as normal (i.e. by pressing the del key)

EDIT: Oh, and this should work for any version of Windows NT as new or newer than Windows XP, not just Windows 7.

There is WiXkill. Another alternative is Windows xKill. Here is the direct download link.