It fulfills a very specific purpose. I have found that I constantly have way too many tabs open. I often click new links to open new pages without closing out older tabs, and it was getting crazy. Somedays I would have 30-40 tabs open.
I wrote this extension so that I could keep a cap. You pick a maximum number of tabs an algorithm (least recently used, least accessed, or oldest) and as soon as you hit the cap it will discard an older tab.
It will never close a pinned tab or a tab that you have never been to (never been to means you haven't been actove on the page for more than 3 seconds)
The source is available on Github here: https://github.com/ccampbell/xtab
Sorry, but this is stupid. I don't "not close" tabs because I'm too lazy and need a program to do it for me. I leave them open because I still need them as a reminder or to do list. So if that tool closes the least recently used tab there's the danger it's also the least well remembered point on my to do list.
An option that would make that application useful would be Bookmark folder where it puts the closed tabs so you can still access them while they don't fill the RAM anymore.