The maniacs who maintain Drools use a variety of tools, but here is some credit where credit is due:

JProfiler - JProfiler provide an awesome profiling tool. Committers of Drools can get a licence for this gratis, so much appreciated. Its IDE integration (Eclipse and IDEA) works brilliant.

Eclipse - Eclipse (3.2) of course ! But some curmudgeons use emacs, I am sure Many others use IntelliJ as well.

Maven - Maven 2 to be precise. Whilst you only see an ant script, its maven 2 behind the scenes managing dependencies.

Tortoise SVN - a good integrated windows SVN client. On Linux, RapidSVN is good (from the same folks). SmartSVN is also a good client.