Personalized ring-tones are a great way to know who's calling without looking at your iPhone.
The Apple store has a bunch of ring-tones available for $2 a pop. But you can save yourself some coinage by making your own, from any song you have have lying around in your iTunes collection.
The team over at iphone-hacks.com have a great tutorial on the process, showing step-by-step instructions on how to make a 30 second ring tone from a MP3 of MP4 (aac) file.
Enjoy!