There are a few wireless driver supports Master mode (infrastructure mode or AP mode), you can manage these interface with iw tools and provide network services for others, just like a wireless router do.
However, many of cards does not support this feature. But you still can use iwconfig to switch to ad-hoc mode, which most of cards support. With ad-hoc mode, the others can directly communicate with you when both agree on the same network name (SSID) and channel.
So, with properly setup, you can create a local wireless network for nearby nodes, or even share your network with others. What I am interested about it’s to share the network with others.
Actually, the modern linux distribution with Network Manager installed has already supported this kind of feature. It’s very easy to setup in Network Manager.
Basically, you just need to setup a ad-hoc network manually, and enable it when you are connected with ethernet or 3g network.
A quick tutorial –