Some humorous ways to end a letter include: "All you need is love;" "Hasta la vista, baby" and "Live long and prosper." These are pop cultural quotes with which many letter recipients will be familiar.
Some letter endings may be inadvertently funny, including those that cross a language or cultural barrier. One such example, received by an editor for Forbes magazine from a travel writer in southeast Asia, was, "Have a wonderful bountiful lustful day."
Some comical signoffs work better in emails. Examples from Forbes include: "Sent from a prehistoric stone tablet" and "Pardon my monkey thumbs." Like many repeated jokes, these are likely to wear thin pretty quickly.