The same could also be used for wireless "keys", like the ones many of us use to lock/ unlock our cars. If you haven't visited Sparkfun yet, do it! LOTS of fun toys to play with. And a great forum for Sparkfun-product-specific questions. (And no, I don't have any connection with them beyond "happy customer"... and if the OP DOES have a more direct connection, I for one feel that the valuable content in his/her post more than makes up for any "advertising" element.)
will take you to the page about the $4 transmitter
Note the following, which is quoted from a user post on that page...
Sparkfun say: "Only one 315MHz transmitter will work within the same location." Only one can work at the same instant, but for datalogging, with message transmissions of a small fraction of a second, many can work in the same space provided that you understand and work around the possiblility that collisions may occur with resulting data loss--so transmit multiple times with varying gaps between transmissions.
Note, oh international readership, also that the rules about what RF devices are allowed vary from country to country. The link above takes you to a 315MHz device. Sparkfun also sell a 434MHz version of the same thing... plus fancier RF modules.
To expand my "key" idea: Fit a transmitter with two square wave generators (two time something similar to the temperature sensor circuit. but with a simple resistor in place of the temperature sensor.) Each "turned on" by pressing a button. The system behind the receiver "watches" for first a brief "send" of the first freq, then a brief send of the second.