You have some data you want to back up. Or maybe take home from the office. Perhaps you want to share some stuff with friends.
These services will hold your data for as long as it's being downloaded - after one month of inactivity the file will be deleted. They supply unlimited downloads (though download speeds are far from uber).
Rapidshare - files up to 50 MB.
MegaUpload - file up to 250 MB.
Zupload - will store up to 500 MB. This one is new and I don't have much experience with it, but as of now it seems most satisfactory.
YouSendIt allow you to store on their servers up to 1 GB of data, but only for a limited number of downloads and for no longer than two weeks. Useful for larger files with only a small number of recipients.
Spread the wealth.
edit: This guy
right here has the definitive list.