I'm not sure if you can call it a memory leak since I don't believe there is a dealloc in Java but my SQL Developer (3.2.10.09) running on XP with Java 1.6.0_11 won't release memory. After running for a full day or two it's taking ~740MB or memory and begins slowing down. It's painful to edit a file when this happens and it's painful to restart but I know I will have to eventually because it will just hang on operations for long periods of time.
I've searched this forum and elsewhere and have found many suggestions including closing worksheets so Java will recognize the unused memory and give it back to the OS but this does not happen with the version of Java I'm running on.
There are other threads in the forum mentioning Garbage-First, so you may also wish to search for those. Also, try upgrading your Java to the latest Java 6 update: jdk1.6.0_37, or upgrade SQL Developer to the latest 3.2.20 release which makes jdk1.6.0_35 available in the primary 32-bit Windows download.
I installed the new 3.2.20.09.87 with the 1.6.0_35 JDK and added the suggested setting changes to the sqldeveloper.conf. I definitely see that memory is being freed up as I close worksheets but not as much as I would expect. I'll work with these settings this week and see how it goes.