Small and easy web server. According to its official web site, it is open source, but its big brother Xitami/Pro (with SSL support), is not.
It is available for a wide variety of platforms, including Windows, NT on Alpha, Unix, OpenVMS and even OS/2, though I have only used it under Windows (running Apache now).
Xitami is quite fine as it is really lightweight (diminutive footprint), really easy to set-up and complete. It has no security problems that I know of and it puts a little 'x' in your system tray... what else do you want? It even has an ISAPI/NSAPI/etc. look-a-like, WSX, some ISAPI support and persistant CGI.