Here's the first X puzzle.
This puzzle either runs inside the web page, as it should be doing now, or you can use the "Float Window" button to popup a new window which you can make bigger or smaller as you want. To fill in a number that you definitely know, use your mouse to left-click on the square and select the number. If you're not sure but want to mark a possibility (for example, the 2 can either be here or here), right-click on the square and select the number. This possibility will be written smaller and grey in the corner of the square, like a pencil mark to remind you. See about the helper for more information on what all the buttons, markers and rollovers mean.