favorite/essential Gems

Having successfully worked through a few tutorials now, I am curious as to what is out there for available Gems. The number of them at http://gems.rubyforge.vm.bytemark.co.uk/gems/ is overwhelming. And as I have a slow dial-up cnx and limited ISP online time, dealing with the lengthy "Bulk updating source index" just to install Gems to explore them isn't really an option for me.
I suppose when I need one, I'll find it, but I was wondering what Gems others were using. Why you like/need them? What they do? etc.

I've never specifically looked for Gems to install before I had a problem to solve. Usually once I start writing an application and a problem arises that I think may have already been solved, I go looking for a Gem. All my apps use a different set of Gems.