Note: CargoLifter is no longer for sale. Its functionality is now included in Feigeist's MailButler, whose link can be found int he following paragraph.
CargoLifter--which is now a part of MailButler, the complete assistant for Apple Mail--automatically uploads your email's attachments to the cloud (e.g., Dropbox or YouSendIt) and adds the corresponding links to the end of your email. Thus, the recipient can decide himself when he wants to download the files and more importantly his mailbox does not get filled with attached files. Your colleagues and friends will thank you!