Openwrt is a Linux distribution for embedded devices like the common Linksys WRT54g router/accesspoint. I use it on two of these nice devices. One acts as my router and also runs digi_ned for my little APRS-digipeater. The other one’s places on the roof for wireless-lan access.
Some time ago Jan Beba and me wrote a little tutorial on how to setup openwrt for a hotspot with the opportunity to access your LAN via openvpn. It’s not up-to-date but maybe useful for learning.
Tutorial