Windows: Windows 7 was supposed to have some rather cool Wi-Fi sharing tools installed, the kind that would let other devices and computers jump onto one computer's ethernet or Wi-Fi connection. A free Connectify beta brings that helpful feature back.
Grab the installer—you might have to refresh once or twice today, as the servers seem busy—and install Connectify, and your system tray gets a Wi-Fi-looking icon. The app is seriously simple to use once it's installed. When you're connected to an ethernet or Wi-FI connection, you can create an access point name, give it a passphrase, and choose which net connection to share, if you've got a few going at once.
Your friends don't see an "ad-hoc" network with limited powers—they see a real access point, served up from a virtual, cloned version of your Wi-Fi adaptor. It seemed to work on a quick home test, though my screen capture powers are limited at the moment. Whether you're sharing a paid connection with your mobile phone or a friend, or otherwise spreading the net magic, it seems like a great route for doing so on Windows, without extra hardware or tricky tethering hacks.
Connectify is a free beta download for Windows 7 only—the virtualised adapters made their way into 7, but this kind of easy front end didn't.