2008 MacPro 2 x 2.8Ghz Quad-Core Xeon , 28GB RAM, drives are SSD.
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.
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.
Indeed, Safari is the only browser that doesn’t render my machine unusable under such use. (Both Chrome and Firefox seem to consume dramatically more CPU and memory for the same load.)
What kind of situation are you running into where it stops working after an hour?
How much memory do you have on your machine?