I find tab-completion an extremley useful feature of my IRC client, irssi, and most Linux/UNIX terminal emulators. I often wish that tab completion of filenames, commands etc was implemented in other software as well.
Eclipse does have this feature, but it is often overlooked – simple open a file in Eclipse, in the Perspective that you are programming in, and hit CTRL + SPACE. You will be given a popup similar to the one below, and you can start typing the name of the method/function you want to call, and it will show you a brief description, and the parameters that it takes, without having to look them up online – how handy is that!
