The folks at WiTopia, a group that sells personal and small-office scale network security tools, are offering their WPA Enterprise service for Wi-Fi networks of up to five users and a single base station at no cost for a year. This is a boon to small networks because it allows you to have high-level security for each user - even if it's just you - without having to create and enter encryption keys.
WiTopia uses WPA Enterprise, which combines the strong encryption found in the algorithm used in WPA (Wi-Fi Protected Access) with individual usernames and passwords. Mac OS X 10.4 supports WPA Enterprise; Mac OS X 10.3 works too, with the latest AirPort firmware and software. When you connect to a Wi-Fi network that uses WPA Enterprise, Mac OS X prompts you for your username and password. If you enter your credentials correctly, the access point assigns you a unique encryption key - no one on the network shares that key with anyone else on the network. This effectively gives you a secure wireless link that can't be sniffed. (Someone can sniff at the Ethernet port if they have physical access, of course.)
WiTopia maintains your information on their servers, which you access in two different ways. To add, change, and remove users, you access their secure Web site and use a simple interface. Your access point also directly access a separate, secure authentication server using a separate password they provide (a shared secret) to confirm a user's login details when they connect to the wireless network. (A WiTopia staffer pointed out that an AirPort Express can become a portable secure WPA Enterprise network: if it's configured to point to their servers, all it needs is to plug into an Internet-connected Ethernet network that assigns it a valid local address, and it's secured.)
The one drawback to their service is that a bug in how AirPort Admin Utility works with the latest few releases of the firmware for the AirPort Extreme Base Station prevents easy entry of the shared secret. It's Apple's bug, and I alerted Apple months ago with no results. WiTopia has a workaround involving Apple's free AirPort Management Utility that's not too inconvenient, and you enter the shared secret only once during setup. They document the workaround on their support forums.
This $29 per year starter package comes with a $29-off coupon, making it free for the first year for up to five users and a single base station. Base stations cost $10 per year extra each, and additional blocks of five users are $5 per year each.