Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
You'll find many Linux-based and/or open source options when searching for a Wi-Fi hotspot solution. Whether you're wanting to give away or charge your visitors for the wireless Internet, you should find something that will work. The best part is that most of these solutions are free -- you don't have to spends hundreds on a off-the-shelf hotspot gateway.
DD-WRT is a firmware replacement you upload onto a supported wireless router. This changes your router's control panel and gives you many more features, including several hotspot solutions: Chillispot, NoCatSplash, WiFiDog, and Sputnik. You might also find a use for the other new features as well. For example, you could create a separate wired or wireless network for your private network with VLANs and multiple SSIDs.