End of May 2012 now - Safari Version 5.1.7 with Lion, 8GB RAM, IMAC.
I'm starting to use "activity monitor" more for monitoring & sometimes imporoving all the memory-eating and occasional hanging with Safari tabs &/or entire windows. Maybe "I'm asking for trouble" but when I work on projects, I often have 3 or 4 instances of Safari open, some with maybe a dozen tabs, one or two with up to 100 or so. Some with video or photo displays, some with forms, but nothing overtly playing or launching as I focus on one or two tabs in back to back browser windows (27" iMac display, G-d bless).
So now I'm finding - really only recently - more and more "gunking up" that doesn't resolve with a restart or PRAM reset even. Once a few browser windows with the tab collection I'm using are back up, all is well for hours or days, and then... freeze city. Here's when I started observing how using Activity monitor can sometimes find a "dead tab" spinning away which I can close and watch RAM get freed up. OK, that's my situation, similar to others I've read.
I tried deleting the .plist for Safari (in library) as suggested. Neither helped nor hurt. Next step: having saved and bookmarked crucial bits I'm working on, I took a deep breath and simply deleted Safari Web Content. Only moments before it was showing well over 100% CPU hogging and 75 thread, and lots of "real memory". The main Safari process was in red, now basically frozen even as I delete some tabs. Now - after deleting Safari Web Content (the open process), I have a very nice surprise: Nothing bad happened to all the open tabs - the currently used "Safari Web Content" has not disappeared. All my windows and tabs are open and "live" - nothing lost anywhere, and.... ta-dah! Suddenly "Safari Web Content" is very well-behaved and so is Safari. Activity monitor shows Safari Web Contet is back, but at 2.5% of CPU (down to .4% as I type), with 11 threads and using 358 MB "real memory". Safari itself, with all the (4) windows open and maybe 200 tabs or so, is now drawing .2 to 1.2 % of CPU as I'm watching and typing, with 12 threads and 186.9 MB "Real memory" use. "Free: 6.0 GB". (out of 8) .
Not bad! So whatever the causes (and it's not just using lots of windows, as I can attest) - whether a "leak" or some function of how RAM is used/released with a journaling system like Mac's, or just one of life's mysteries -
I did my little "scientific test" at last and seem to have found a very easy, harmless, solution in simply deleting the "web content" process through the activity monitor. It immediately creates a new one, and everytyhing is suddenly 'like new" with tons of RAM and CPU space to spare, even with so much Safari content actively displayed.
Thanks to all. Lots of good references and tools and importantly, I know this is not an imaginary problem! Seems pretty common - still. Well, maybe this will work for you, too. No rebooting, new software, or anything else required. So it seems so far. (Fingers crossed.) If in doubt, of course, save/backup. But for me, this worked!