Most devices labelled "900MHz" (or 2.4GHz or whatever) do not operate at precisely this frequency. They will have several channels nearby the quoted frequency. Maybe the 4 channels are 903MHz, 904MHz, 905MHz and 906MHz or whatever. The channel spacing is going to vary widely depending on the bandwidth required for the transmissions etc. 802.11 WiFi channels for example are spaced by 5MHz, with the band centred at 2.442GHz (so channel centres at 2.442, 2.447, 2.452 etc.), but the transmission is something like 20MHz wide, so neighbouring channels overlap and interfere with each other. This is frequency-division multiplexing, and is probably the simplest and cheapest scheme to implement, and is continuous in the time domain, so most consumer ISM gadgets do it this way. It gets more complicated when you think about something full duplex (transmit and receive simultaneously) like a cordless phone - it can't transmit on the same channel it's receiving on, so it might use a separate uplink and downlink channel. Usually spaced by some fixed spacing much larger than the channel spacing (for filtering). For something like a cordless phone, for example, you might have 4 uplink channels at 902MHz, 903MHz, 904MHz and 905MHz while the corresponding downlink channels are +10MHz, so 912, 913 etc. A cordless phone probably wouldn't have a 1MHz channel spacing, but you get the idea.
There are other techniques too, such as code division multiplexing (aka CDMA) you have probably heard of, which relies on each transmitter modulating their transmission with a special code that allows fancy math and signal processing to extract multiple overlapping signals. This is used in modern cellular networks and GPS, among others. Time division multiplexing is also possible, and is more akin to frequency division multiplexing in that each transmitter is assigned a short time slot (by some arbitration system), where it is the only transmitter on the channel, in this way multiple transmitters can share a channel and not overlap. I believe this is used in Bluetooth, DOCSIS and some other systems. It was also used in many early wireless digital networks like GSM, but these have mostly moved on to CDMA now.
Anyway there are lots of ways to skin this cat. Chances are your consumer gadgets are just using a different frequency for each channel. Some fancier things, like the better cordless phones, are using spread spectrum techniques and probably CDMA. In these schemes, the transmitter makes a wideband but very weak transmission (intentionally near the noise floor), and the code known to both transmitter and receiver is used to extract the signal from the noise.
I don't have any good articles at hand, but some of the wikipedia stuff isn't bad. Here is probably a decent place to start:
http://en.wikipedia.org/wiki/Channel_access_method