A web service can be written in any language. A web service is a program that takes request and returns response (xml or json) via http protocol. You can use a web server like Apache or lighthttpd to handle the http(s) and multithreading for you and write a simple script to do the actual work. The script can be written in anything - php, perl, python, shellscript, cgi c++, free pascal cgi etc.
Of course, You can write everything on your own by using TCP sockets, but this is not your goal I guess.
For FOSS I'd do it in php, because it`s easy:http://davidwalsh.name/web-service-php-mysql-xml-json
If I want it compiled, i'd use FreePascal as in this guide: http://leonardorame.blogspot.com/2010/02/web-20-programming-with-object-pascal.html
Or If I prefer C++, I'd use QTCreator with this guide: http://libqxt.bitbucket.org/doc/tip/tech-web.html
