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.
Just keep tabs open, they eventually start chewing up hundreds and hundreds of MB of ram each, and slowly grinds itself into the ground. Relaunch, it's snappy again.