Usually, you log onto an FTP server with a user name and password. Nevertheless, many sites offer a guest login. Type in anonymous as user name and your email address as password.
FTP servers supporting anonymous login usually allow users to download files which are publicly accessible without restriction, e. g. shareware programs, image collections, help texts, etc.
Note that there is a special etiquette for public servers:
Do not stay connected longer than necessary to avoid that server performance is reduced.
Do not download too many files.
Read the "readme" files which are found in most directories; they contain information on the directories' content.
A further characteristic of such (public) servers is that users may view and download files, but not upload them.