Interesting service I found (thanks to Stacy Gardner): www.wheresmycellphone.com.
Lost your phone? Pull up the home page, type in your number, and it'll call the phone for free. (If you actually listen to the call, it's a 15-second advertisement for some other product, which presumably pays the freight for the website. That, and the ads on the homepage.)
I've done this trick any number of times with my landline in order to find a cellphone. But this might work well if you don't have another phone handy. Kinda neat, actually.