Windows httpd may be used without charge for personal and educational use. For commercial use there is a one-time fee of $99.00.
Windows httpd is based on the Winsock standard that supports most TCP/IP clients for Windows. You can learn more about Winsock in my paper "Windows and TCP/IP for Internet Access."
At first glance, Windows httpd is an intimidating package. The ZIP file unzips into three dozen subdirectories containing over 400 files totaling more than 2.0 MB. However, the server executable itself is only 244,240 bytes. The remaining files contain extensive documentation, images, sounds, and support software for uses more sophisticated than simple file serving. Initial setup is straightforward.
MacHTTP must be registered following the initial 30-day trial period. The cost for educational use is $65.00 per server machine. All other users (commercial, government,) are charged $95.00.