I like the Regex and SQL plugins. Which are your favorite ones?
The SQL Query Plugin
(Yes, it's mentioned in the question, but to get comprehensive results for this poll, it should be included in answers too.)
This whole thing should be a community wiki, btw.
IDETalk. And not for chatting but because it makes it easy to share stacktraces send code pointers and compare code with colleagues without having to first commit it.
I'm not a heavy user, but some of my colleagues like this plugin a lot
Scala for NetBeans is stable for 2.7.3, 2.7.4, 2.7.5 and coming 2.7.6. The nightly built is target to 2.8.0, also stable enough.
For me the best IntelliJ plug-in is the JFormDesigner plug-in (even if it's commercial). Even some of the Jetbrains guys recognized that this plug-in is much more polished than their own GUI Builder.
As in general, I think the best IntelliJ plug-in is the one that YOU as a user will write :). This would be also the best for IntelliJ's future. The more users would write more plug-ins, the better :).
JFormDesigner is the most advanced and polished application that can run under IntelliJ as a plug-in.
Identifier Highlighter is an essential.
I can't imagine developing without being able to "Alt + Shift + Up Arrow".