I have been using Gmail a whole bunch lately and getting deeper into the advanced functionality. One of my favorite features is the search operators. These Ginsu Knives allow you to both find and filter email.
I found a help page that gives a great overview of the search operators that I find really useful. It's not hard to find the page but I thought I'd link you to it anyhow - Click Here.
Example:
from:Dave(friday funny) - returns email that is from Dave with the words 'friday' and 'funny' in the subject
Until I started using Gmail more I had no idea these features are there, and I think most folks are the same way. Finding email is my second biggest feature requirement for an email and this makes my life easier.