TI Invaders is in my opinion *the* best space invaders clone ever released.  It was one of 2 games I got with my TI99/4A computer I received at Christmas in 1983.

The thing I like best about it is that it made you carry on and play ‘just one more game’ as the invaders would change in type every 2 levels.  Me and my brother would carry on playing as we were intrigued to see what the next ones would look like.

There was a mini game after clearing each wave where you would shoot at a saucer, hit it and it would get slightly smaller and smaller and smaller until you missed.

I still have my TI99/4A and fired the game up the other day and its as enjoyable as it always has been.