It's so easy to take Address Book for granted, and yet that simple utility has two absolutely killer features: big phone numbers and simple envelope printing. These features do exactly what their descriptions suggest: display phone numbers in Very, Large, Type -- and print envelopes, complete with your personal return address.
To zoom a phone number, control-click or right-click the number you want to see and choose Large Type from the contextual pop-up. Address Book zooms that number way out, allowing you to see it while dialing, even if you're not standing right next to the computer. I find that it's a lot easier just to show the number to my husband than try to read it out to him. Your spousal success rate may vary.
To print a properly formatted envelope, just insert a standard envelope into your printer. Make sure the address side (and not the flap side) faces the toner cartridge. For me, that is flap side down. My printer, which is pretty darn standard, has an envelope feeding clamp in the single sheet feed. I pop open the single sheet access, adjust the clamp inward to match the envelop size and feed in the envelop flap side down.
In Address Book all I need to do is select a contact and print it. (File > Print). Make sure you've chosen Envelopes from the Style (it's my default), and that Print my address is selected so the return address is properly added to the envelope. If you're using an envelope that's not a standard business layout, choose the proper layout. Address Book supports most common envelope styles used in North America, Japan, and "International."
Address Book can also print out mailing labels, lists, and a pocket address book. See the Style pop-up for details. For the free contact manager that comes with the OS, it's got moxie.