The original 960.gs written by Nathan Smith still stands as the default grid solution in most cases. Sure there are more involved grid systems out there that address any number of needs, but the original 960.gs when deployed will quickly solve your layout problems.
Quick and easy to deploy. Easy API to get used to and anyone can get quickly trained up.
It's not responsive. It was built way before the concept of responsive was invented. It replies on using presentation classes in the DOM and when applied, it is somewhat rigid.
The following deck is a presentation Nathan gave back in 2012 in Oklahoma.