Windows: Remember Microsoft's Virtual Wi-Fi project for Windows 7? Well the project is officially dead, but the source code is still available, and free application Maryfi takes advantage of this to provide a software router that can share single-serve Wi-Fi networks such as those used at airports and hotels.
Tethering and mobile hotspots are great if you're a frequent traveller or if your company is footing the bill. I travel to one or two conventions a year and usually tether to my phone's connection for Wi-Fi, but I didn't get a very good connection on the 37th floor and had to pony up $15 for one day of unlimited Wi-Fi. Even worse was that the two friends I shared the room with used my laptop when they needed to check their email. If I had Maryfi, I could have just shared my hotel connection with everyone in the room.
Maryfi can share any type of internet connection on the host computer; other Wi-Fi enabled devices can see and join the Maryfi hotspot which can be secured with WPA2 encryption. Another use is to setup Maryfi as a repeater for your home router -- this can help you stretch connectivity to a device just outside your current Wi-Fi range.
Maryfi is a free ad-supported program for Windows 7 available from the developer link below.