FWIW....as a parent of a 5 year old AND a 6 year old (13 months apart)....
Many of the games mentioned are great. We've also got iWriteWords, Mental Case (a flash cards app that I have their sight words in), pocketphonics, Flash Cards, Koi Pond, Frogger and TicTacUlt (tic tac toe ultimate).
They've outgrown a few others....but those keep them interested.
And to the naysayers.....I'm one of the biggest on 'parenting old style' (if you will). Grew up with no video games, computers, not even cable tv. Had to go to my neighbors/best friend's houses for all that (and really didn't get into it).
But, it's a different teaching world now. Both my kids have brought home CDs that have the exact text book material used in class for math and spelling. The teacher sends weekly 'what are we doing' notes home to let us 'follow along' with the class. They have computer assignments weekly for math. For the kindergartner, it's just the CD text book and I mostly print out pages, but there are follow along teaching and doing videos also on the CD. In fact, the 1st grader has 3 weekly math assignments that are done ONLINE and the results are sent to the teacher. This is all from a public school district. I'm pretty impressed, and luckily my kids are enjoying it and love to go to school (fingers crossed it lasts!).