802.11b is 1, 2, 5.5, and 11 Mbps in the 2.4GHz band. 802.11a is 6 Mbps through 54 Mbps, in the 5GHz band. 802.11g is the same thing as 802.11a, but in the same 2.4GHz band as 802.11b, plus several tricks to allow coexistence with 802.11b. And 802.11n is a set of enhancements to all of these, working in both bands.
So, in the 2.4GHz band, we have 802.11b only hardware (mostly obsolete), 802.11b/g hardware (the most common; automatically choses from the set of 802.11b and 802.11g speeds the best one for the situation), and 802.11b/g/n hardware. In the 5GHz band, we have 802.11a only and 802.11a/n.
Since almost everyone can use the 2.4GHz band but not everyone can use the 5GHz band (wireless hardware which cannot do 802.11a is still common), most public networks will use the 2.4GHz band. Since there is no thing as 802.11g without 802.11b (even a pure 802.11g network will still use the 802.11b speeds, and the beacons use the slowest speed; disabling 802.11b in the access point only disables a few tricks for compability with hardware which does not understand 802.11g), all networks you see in the 2.4GHz band will be using 802.11b (and usually also 802.11g).
That said, there are several advantages to greater speeds you might not be seeing:
- Lower energy use. With higher speeds, the the transmissions are shorter, meaning the most power-consuming part of the wireless hardware (the radio) will be used for less time.
- Greater reliability. There are two kinds of interference: continuous interference (where a slower speed can help; this also includes weaker signal, since the signal is weaker but the noise is the same) and sporadic interference. A shorter transmission has a smaller probability of coinciding with a burst of interference, so faster speeds are better for sporadic interference. And all interference from other wireless LANs and Bluetooth would be sporadic.
- Lower latency. One component of latency is the time needed to transmit a full packet.
- Better spectrum sharing. If you are not the only wireless user in the area, shorter transmissions allow more time for everyone else to transmit. Unlike twisted-pair ethernet, the radio is a shared medium, meaning the 54 Mbps maximum of 802.11g is shared among everyone.
And while most focus on the speed enhancements of 802.11n, it also has a few tricks which can enhance the signal reception (improving reliability and as a side effect allowing for better range with the same power). But given what I just said, even pure speed enhancements are a good thing, even if you are nowhere near saturating the 802.11b speeds.
The best papers I have found so far about 802.11n: