1 - Search for a term you're interested in finding on FTPs:
inurl:ftp -inurl:(http|https) "Microsoft Office"
2 - Search for everything Google has indexed from a particular site's FTP:
inurl:ftp -inurl:(http|https) site:gowinnt.microsoft.com
3 - Search for FTPs from educational Web sites only:
inurl:ftp -inurl:(http|https) site:edu
4 - Search for a specific file type on FTPs:
inurl:ftp -inurl:(http|https) "Microsoft Office" filetype:pdf
5 - One fun search query to rule them all:
inurl:ftp -inurl:(http|https) site:gov | site:mil filetype:pdf | filetype:ppt | filetype:doc "Top Secret" | "Classified Information"