[The Grammarian]

I'm waiting for the Ion Cannon available for home defense.. (as per Planet Hoth defense in The Empire Strikes Back)

I'm looking more forward to seeing ion cannons deployed on satellites by the US military as alternatives to tactical nukes, a la the Westwood Studios PC game Command and Conquer.

[Junior]

Ions are charged particles (having had their electrons stripped away for easier manipulation). Charged particles let loose in a vacuum will quickly repel one another and dissipate. Ion guns in orbit won't work.

However, charged particle beams, with a little help from a tunneling laser, hold together a bit longer in an atmosphere and could be used as weapons, which is what the Directed Energy Weapons programs of the former Soviet Union and the United States were working on.