Which window has current focus?
Solution 1:
What you want is libwnck (if you're just interested in windows) or libbamf (if you're interested in windows and the applications that own them).
Solution 2:
Another thing you can use is xdotool:
xdotool getwindowfocus
would return the Window ID of the focussed window, and:
xdotool getwindowfocus getwindowname
would tell you its name.