ICS Control is a GUI front-end for Internet Connection Sharing, a component of Microsoft Windows Mobile. It can be configured to do a variety of things including sharing your cellular internet connection over an adhoc WiFi network.
Requirements: .NET Compact Framework 2.0 and Windows Mobile 5 or later with the IPNAT driver included (most WM6 builds have this built-in). The "Internet Sharing" application that some phones have is not required, but IPNAT is.