The most important of these were 802.11a and 802.11b, both completed in September of 1999. 802.11b defines up to 11 Mbps in the 2.4 GHz. unlicensed band, and 802.11a specifies up to 55 Mbps in the 5+ GHz. bands. Both of these chunks of spectrum are available in various parts of the world, subject, of course, to local regulations. .11b was first to market since it was easier to engineer products based on it. But .11a products are today plentiful and much less expensive than they used to be, and .11a's higher throughput and more available radio channels (12 vs. the three non-overlapping channels in .11b and g) make it attractive in enterprise and consumer-electronics applications. .11b was recently augmented by 802.11g, which essentially takes the higher-performance technology of .11a and applies it to the 2.4 GHz. band, while maintaining backwards compatibility with 802.11b - well, sort of. One can't operate 802.11b and 802.11g simultaneously in a single channel because they're incompatible. A "protection" mechanism makes its possible, however, for the two to coexist in an orderly manner. Regardless, the future belongs to multi-mode client devices that will eliminate the need for the user to worry about any of this - the infrastructure will automatically determine which PHY to use at any given moment in time, with features in the MAC and above allowing connections to be handed off dynamically between a, b, and g.
A number of other extensions have also been approved in the past few years. These include .11c, which specifies WLAN bridging, .11d, which extends .11b to work in more countries, .11f, which specifies interoperability between access points of different manufactures, and .11h, which provides the dynamic frequency selection and transmit power control required for .11a operation in Europe (but is a good idea everywhere regardless). Still in process are .11j, which will quantify the requirements for operation in Japan, and .11k, which will provide a standard mechanism for access to specific radio resource parameters in the interest of optimizing the radio link.
A few other major (and very visible) efforts are also still underway. These include: