The Javalobby has a great article called Eclipse: Managing Multiple Eclipse Installations that shows you how you can keep one plugins directory and have it work for multiple eclipse installs.
Since I do a lot of Java 1.5 development, I have to use Eclipse 3.1, and I keep updated with the latest milestones. This means I am installing a new version of Eclipse every few months, and I was reinstalling my plugins each time - I knew there was a better way, but I never got around to finding it.
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I've just started using Eclipse, so I've not much experience to bring to the table. However I had version 3.0.2 installed, with several plugins. I tried installing the WTP (Web Tools Platform) and could not get it to work. Someone in the newsgroup then told me that I needed version 3.1M6 to make WTP run. Having no knowledge on how to update the Eclipse version I downloaded it, and then just unpacked it into my "old" Eclipse directory. When I then opened up Eclipse it was the new version (3.1) that opened, but all my old projects and plugins was still intact. And WTP even worked without trying to installing it once again under the new version. I probably did the update the "wrong" way (knowing no other), but thought I'd share my experience with it.