How Can I Tell What Search Engine Sent a User to My Site?

I want to know what search engines to focus on. Is there some sort of script that will allow me to do this?
Edit: I would prefer not to use analytics, I want this website to be as light as possible. I am looking for a stand alone script.

Pretty much all stats programs do this (awstats, ect.)

You can easily figure it out from Google Analytics data.
Once you've installed their code and its fetching data, all you need is to visit the traffic sources > Search Traffic (non paid) and then you can check out the visits sent by each search engine...
Hope this helps :-)