I personally use Eclipse (
http://www.eclipse.org/) with the Aptana plugin. Eclipse has great project management, along with a host of plugins that let you go from PHP to Ruby to Java to whatever you need tomorrow. And, when you get to know an ide, you can work all the more efficiently. That said, it is heavy. It runs on JVM, so it has a big memory footprint. Still, I love it.
Previously, my favorite was phpDesigner2005 (the more modern versions tried to do too much, were nagware, just didnt cut it). Now, when I am doing light editing (i.e. not working on a large project) I use bluefish (
http://bluefish.openoffice.nl/).