Ask HN: Good app ideas to learn a new platform

So I wanted to start learning iPhone development last night and I didn't really know what to build. I thought it would be nice to have a list of little projects in order of increasing difficulty for learning mobile development, web development, game development, and desktop development.

Got any ideas?

If we get a good list, I'll put them up on a website or something like that.

Here are mine:

For game development:

1) Blackjack (console)

2) Tetris

For web development

1) todo list