http://www.scintilla.org/
does code folding
draws little lines that lines up if brackets, etc
does syntax coloration for more stuff than i knew existed
http://www.chami.com/html-kit/
syntax coloration
far more plugins than you will ever need
http://www.maguma.com/en/lightdownload.html
full blown IDE (does auto-completion for your own classes/functions as well as php's built in stuff)
syntax coloration (whoo!)
phpdoc support (pops up tooltips, and... eh. similar to javadoc in java)
http://phpeclipse.sourceforge.net/
plugin for eclipse, does same things as maguma, but you get all the power of the eclipse platform
http://www.xored.com/products.php
similar to the above, slightly inferior (IMO), but the two both have slight advantages over the other
there are lots more...
