Fetchmail is a program that retrieves email from a remote computer via POP (POP2, POP3, RPOP, etc.), IMAP, and ETRN. After it retrieves the email the program forwards it to the local MTA (be it sendmail, qmail, or whatever). This makes it very useful to those who use *nix and connect to the internet via a dial-up.
Under certain circumstances it can be unreliable, though. If your computer unexpectedly restarts/shuts down before fetchmail is able to forward it to the MTA (and you've configured the program to delete the mail on the server after it is retrieved) then your mail is lost.
Fetchmail is maintained by Eric S. Raymond.