Best FTP search engines 
If you look for some files and you know they are located inside an anonymous FTP server, these FTP search engines will help you find it.
Despite criticism and some premature obituaries, FTP is far from dead. It's true that the golden days of FTP are long gone but it's still used by millions of sites.
FTP is a 40+ years old Internet protocol but it has been redefined multiple times and it's still used to transfer files between computers on a network. The users connect to FTP servers using FTP clients like BriskBard, that allows them to upload, download, rename and delete files and folders on the server. It's usually necessary to login to the server but some of them allow public access and they are known as anonymous FTP sites.
FTP was extensively used in the 90's but with the increasing popularity of the web and P2P services, its role has changed. For example, 25 years ago most of the shareware and freeware software titles were distributed using FTP sites. Nowadays, software download websites and the bittorrent protocol are the preferred way to do that.
Many companies, government institutions, universities and open source projects use FTP sites to transfer all kind of files between their members and users, many applications use FTP to transfer files in the background and many web developers use it daily.
According to the IEEE Computer Society, there are more than 13 million FTP servers and about 1.1 million of them allow public access. This is just a drop in the bucket compared to the 1,800 million websites that Netcraft included in their latest web server survey but it's still noticeable.
These FTP search engines allow you to search for file or directory names. They can't be used with complex search queries like "why is the sky blue" because they only compare the text of the query with the file or directory names.
This is the list of FTP search engines, in alphabetical order :
Archie : The University of Warsaw has a webpage to simplify the use of an archie search engine. Archie is a service that indexes FTP sites and it's considered the first search engine in Internet. Usually you can use archie with a telnet client but this website makes it much easier to use while keeping many of the advanced search options.
Filemare : Filemare is a powerful FTP search engine that allows you to search by keywords in a server, city, state, country, region, continent or even by file type.
Filesearching : This is a Russian search engine for images, videos, music or any kind of files found in thousands of FTP servers. It has more than 31 million files in its database and it allows you to make advanced searches with multiples parameters.
Filewatcher File Search : Filewatcher is an advanced file search engine that not only lets you search individual files but also files inside packages (*.deb, *.rpm, etc). It allows you to search files using their names and contents. Filewatcher has more than 2,000 million files and more than 5,000 FTP servers in its database.
FreewareWeb FTP Search : FreewareWeb is a freeware download website that includes an FTP search section. It has thousands of FTP sites in its index and you can search images, videos, music and any other files using filenames or keywords. It's a case insensitive search with advanced search options.
Global File Search Engine : This search engine lets you search images, videos, music, disc images, folders, compressed files and regular files. It has 166 million files and more than 2,200 FTP servers in its database.
Google : Google can also be used to search for files inside FTP sites if you add a few search operators to your query. The search operators tell google to show the results that have ftp:// in the url and hide the results that have http:// or https:// . For example, if you want to search the word galaxy, you would have to type galaxy inurl:"ftp://" -inurl:"http://" -inurl:"https://" in Google's search box.
Mamont : Another Russian FTP search engine with the largest database so far. It has an index of more than 3,700 million files found in 10000 FTP servers.
NAPALM FTP Indexer : Napalm has more than 949 million files and 14,000 FTP servers in their index. It uses a crawler that updates their database every 2 to 4 days, giving higher priority to the most frequently accessed servers.
Wenjian.net defines itself as a file encyclopedia and it includes an FTP search engine. The user interface is mixed in Chinese and English and it doesn’t tell how many servers are included in the data base.
Some of the FTP search engines in this article let you download the FTP servers list used in their databases but if you need a longer list, take a look at these websites :
openftp4 : This list contains 796,268 anonymous FTP servers from a total of 18 million servers. It's a 217Mb raw log but it has the biggest and most up-to-date list of anonymous FTP servers I've seen so far.
ftp-sites.org : This list has 5,593 FTP servers and the last time it was updated was in 2003.
shodan.io is a search engine for Internet-connected devices but if you search for "anonymous ftp" it will show you a list of more than 900,000 FTP servers.
Updated January 11th, 2018 to add the latest servers.