u need and ftp daemon. i use proftpd just because it is really easy to use. once u download and install there is a config files /etc/proftpd.conf (i think, cd to /etc and look around). if u want to disable anonymous login it is in there as well as port changing(if u need to, my isp wont let me use the default ports). there is just one other file that u have to change (dont remember what it is though, sorry) that has a list of all of the ports and the associated protocols, just change that if u need to. then all you have to do is (for anonymous ftp) type in the address bar ftp://ipaddress:port
(if not the default) and ur good. if u disable anonymous ftp u need to type in ftp://username:password@ipaddress:81
(note that the ipaddress can be replaced with a site name, ex: www.google.com
). u can also use an ftp client to download like gftp. the make sure that u go into the services menu (i know there is one in fedora, not sure of others) and check the box next to the ftp daemon ur using and save. then it will start the ftp server when u start the comp. you should know that the usernames for linux are the ones it looks for as ftp logins when using non anonymous ftp. u cannot use root and in the config file it has the user and group that is allowed to ftp in. for some reason proftpd allows only 1 login. i just give out that login whenever i want someone to be able to download and then change the pass as necessary. there is an option that supposedly limits people who ftp in to their home directory, but whenever i use that it causes the whole service to not work. you should also know that people that ftp in have access to all files and can change/delete their files. there are a number of security flaws here so be careful. i dont know what they are, but i know they are there.
have fun, i hope this helped some.