I wonder if it's something those sites are doing in JavaScript, because I have the same experience as the other poster. I keep tabs in Safari open for weeks at a time without any issue or noticeable memory increase.

However, I do know there are a handful of web app type things (Google calendar and possibly Gmail are coming to my mind) that seem to leak memory and cause issues if left open.

Edit: eridius mentioned Google Docs. I'm pretty sure that's what I was thinking of.