If you ever log into your Minecraft client and go to your Multiplayer server list only to discover that the list is completely empty, immediately close Minecraft and then browse to your .minecraft folder and look for a file named:

servers.dat_tmp or servers.dat.tmp

Simply rename this file to servers.dat, allowing it to overwrite any existing file of the same name, and then restart Minecraft. Your multiplayer server list should now be restored.

You may also want to keep a copy of the servers.dat file somewhere safe as a backup as the file can sometimes (albeit very rarely) disappear without leaving the .tmp file copy of itself.