I dunno about free, but the only game I have on my K750i is Gridrunner++ - http://llamasoft.co.uk/gridrunner.php - which is great for bus journeys or situations where you forget your DS or PSP.
Avoid the mobile version of Lemmings, no matter how tempted you might be.
Avoid the mobile version of Lemmings, no matter how tempted you might be.