Ultimately icons serve the functional purpose of leading eyes towards particular bits of information and/or actions on websites.  Of course, that's not the only reason to use them on your site - the right selection of icons can also add a certain overall feed to your website.

We *love* the Function Icon set - in fact, the icons you see on plantseedling.com pretty much all come from this set.

Released completely for free last year, it features 128 icons measuring 48 x 48px that feature a good number of functional web actions but in a warm and friendly way.  The icons are slightly glossy but don't chase an obviously over-done OSX look like lots of sets on the web.

Download the set to use on your site here:
http://wefunction.com/2008/07/function-free-icon-set