Some month ago I started testing an idea for using the second tab key 
for completion. I think I told about the idea here then.

I found the original idea on EmacsWiki. I have been trying to generalize 
this quite a bit.

In the attached file there is a minor mode called tabkey2-mode that 
implements it. If tabkey2-mode is on then the first tab key does 
whatever it use to do when tabkey2-mode is not on. The second tab key on 
the same row will do completion.

Since there are many kind of completions tabkey2-mode also lets you 
choose completion type. This is bound to shift tab.

Any thoughts or ideas?