Until JetBrains released IntelliJ 8 or 9 I went nuts without the Identifier Highlighter plugin (highlight other uses of the identifier at your current caret placement), I also went to a talk last night where John Smart recommended Infinitest (runs relevant unit tests whenever you alter a file) which seems like a great plugin.
Not sure if it counts, but I also use Pulse to manage my Eclipse installation, having spent an entire evening unsuccessfully trying to get various plugins to play nice.
I'm interested if anyone has any killer recommendations, as I'd never heard of Infinitest, and I wish I'd known about it sooner. To me things like findbugs plugins seem to have limited utility, as you need to remember to run them, and seem to fit better as part of a CI server, so I'm more interested in either productivity or realtime code quality improvements.