Modern operative systems have built-in FTP servers, like MacOS X and Windows 7. 

For older versions of Windows, you can use a free FTP server application like FILEZILLA.

If you want an always-on, configuration free FTP server, you can activate a free account on an 

FTP hosting site. That way, you don't have to configure your computer and you can access 

your shared files everywhere (even from cellular network). 

One of those FTP hosting sites is THIS ONE.