The holster format was set up & used a lot by the elite spec ops troops of the UK's SAS or Special Air Service. The 22nd SAS is the counter-terrorist section of the SAS(like our US ACE/SFOD-1 "Delta Force").
I've never owned or used SAS type leg rigs but they are very popular.
Safariland, Blade-tech, Bianchi, Blackhawk SERPA all make drop leg/SAS holsters. The Bianchi UM92II series is good because you can quickly modify the holster from SAS to belt to tanker/shoulder in the field.
The offside leg can also tote gear but to be honest, unless you want the nickname: Tactical Ted or Snake-Eyes(GI Joe), you may want to use caution picking these rigs.
www.bianchi-intl.com www.safariland.com www.blade-tech.com www.blackhawk.com