I've immediately jumped on the new 6.5 release of the Netbeans IDE when it was released with much fanfare about its PHP support. Netbeans is a free to use IDE sponsored by Sun. I've used it years ago to edit XML documents, but did not look back on it ever since. Now it does indeed have nice PHP syntax highlighting, context sensitive code suggestions and debugger support as you'd expect from a PHP IDE.
Drupal related posts by Gábor Hojtsy.