As much as I enjoyed reading the transcript of Steve Jobs' commencement address to the graduates at Stanford (here's an audio version), I preferred the similar** sentiments of David Foster Wallace in his Kenyon College commencement address:
The point here is that I think this is one part of what teaching me how to think is really supposed to mean. To be just a little less arrogant. To have just a little critical awareness about myself and my certainties. Because a huge percentage of the stuff that I tend to be automatically certain of is, it turns out, totally wrong and deluded. I have learned this the hard way, as I predict you graduates will, too.
As in his writing, Wallace has a knack for depicting the world as a pretty messy place that one must navigate with a certain amount of uncertainty in order to really experience anything, which, for me, holds a little more truth than Jobs' "grab the tiger by the tail and live, dammit" thoughts.
See also some other graduation speeches:
Conan O'Brien's Harvard Class Day 2000 speech
Will Ferrell's Harvard Class Day 2003 speech
Jon Stewart's William and Mary 2004 commencement address
** Yeah, I know, all commencement addresses are pretty much the same.