Sudoku is by far one of the most popular games to play on a train or bus journey and as such, the format has been cloned millions of times for mobile device (maybe I'm exaggerating a bit).
This game, simply called Sudoku, is one of the most in-depth incarnations of the puzzler I've seen and gives you lots of freedom over how you play it. When you start you can choose to set up the game board yourself, placing numbers on the 9 x 9 grid simply by clicking on a square and selecting one from the pop-up keypad, or using the Palm handwriting pad. It's great because this means you can effectively set the difficulty level of the game manually (there's also an auto option if you can't be bothered to do it yourself).
Once you're happy with the layout you press 'Go' to start playing. One of the most innovative things about the gameplay is that you can choose whether or not you want to 'set' a number. So, if you're not sure if the number will work in a certain square but want to mark your intention to play it you can do so by unselecting the 'Set' option. The number is then displayed in minature and you can add multiple numbers to one square to play around with possibilities.
There's lots of ways to customise the Sudoku board to your tastes, although strangely you can't play around with grid sizes. The only other niggle I found was that the handwriting recognition is a little iffy.
On the whole though, this is an enjoyable Sudoku clone with plenty of options to keep you amused for a long time.