Notepad? Srsly? Booourns. Only masochists code anything with notepad.
On Windows:
PS-Pad:
http://www.pspad.com/ | My personal fav
Notepad++:
http://notepad-plus.sourceforge.net/uk/site.htm
Ultraedit:
http://www.ultraedit.com/ | best of the bunch, but costs money
Also good if you want a full on IDE:
Eclipse:
http://www.eclipse.org/pdt/
Zend Studio:
http://www.zend.com/en/products/studio/ | Eclipse addon by the makers of PHP.
Not really relevant to the thread topic, but in case anyone cares, I use Textmate and Coda on OSX. Both are lightyears better then any text editor I've used on the Windows platform.