If you're just looking for a grid document to use for a digital playmat (I do this all the time), you can easily create a flawless one in a few simple steps:
1. Make a document in increments of 50 per inch (so, let's say, a 2000 by 2000 to make a 40x40 mat)
2. Turn on photoshop's built in grid (in the View) menu
3. Set the size of each grid square (in Preferences) to 50
4. Viola!
You can alter the X-per-inch to any size you feel like for the same result at a higher or lower resolution.
Elton: Assuming you need something that can be printed or displayed on-screen, I have a good methodology that can do what you need. I've also uploaded a template you can download and customize if you prefer.
Hope this helps...
Linkage: Styleable Grids in Photoshop