Any should work. What you are suggesting is used everywhere, TV, radio, mobile phones etc. One transmitter's signal can be picked up by as many receivers as are in range.
Single transmitter with Multiple receivers is the basic concept of FM and AM. Most commonly, in a TV, AM is used to broadcast video while FM is used to broadcast audio signal.
Your original question was "one transmitter, many receivers" which was correctly described above as a broadcast system.
Now you switch to "many frequencies" which changes the problem completely.
Sparkfun sells some simple rf transmitters but you might want to look into bluetooth as it seems a lot more reliable. The ones sold on ebay might be a decent choice. I have two but I haven't used them so I can't say for sure they would work in your case. In fact does anyone know if such transceivers are capable of sending to multiple receivers?
I think the OP wants to use a single transmitter to send several unique signals, each of which will be received by only one of several receivers. This can only be done with a complex system of sub-carriers and/or encoding-decoding.