Not in a/b/g as far as I know (assuming you're asking about bandwidth available to individual clients rather than total network bandwidth) but there is a non-standard Super G
extension from Atheros that uses channel bonding (along with frame-bursting and compression) to increase bandwidth. It was criticized for causing interference to existing b/g networks and other users of the 2.4 GHz ISM band.
There is another 802.11 non-standard extension
that uses only frame-bursting and compression techniques within existing channeling to increase bandwidth: 125 High Speed Mode
Current focus is on MIMO
's Spatial Multiplexing
technique that increases bandwidth and range by transmitting unique streams through individual antennas on the same channel and some clever code at the receiver to recover data from the individual streams using their spatial signatures. This will feature in 802.11n