Hi,
I personally find
JProfiler very useful. It is not free (though cheaper than JProbe I think) but there is an evaluation version.
I did find an Eclipse plug-in, but it seems easier (for me) to execute JProfiler standalone (it controls startup and shutdown of my app server - Oracle OC4J in my case but many are supported).
I don't use it too frequently (less than I should) but the value gained when I do is well worth the license cost to my company.
One of the initial things I liked (having used an older version of JProbe) was just how easy and quick it was to get JProfiler up and running with a real application providing results that I could interpret easily. We are talking about 15-20 minutes from initial evaluation copy download. Through further usage more features become evident, but this ease of getting to a reasonable starting point was a big point in it's favour for me.
Of course, JProbe is excellent, and there are others. When I looked around (about a year ago) there didn't seem to be any open source / free to use products that gave anything like the value of the commercial products, but I may have missed something and things may have changed.
Cheers,
Simon