I ordered the flash cards on this Website and they have really helped me in learning to play by ear.
Only $6.00 (including postage) and you get laminated cards with all of the keys (one key each side).
Most all songs in the world can be played using only the chords across the top (Primary Major Chords and Primary Minor Chords).
I look at a song and note what chords are there and verify that they are the same as the top of the card. I then try to play the song (familiar tunes) looking only at the card. It is getting much easier to play by ear.