The D&D Dice Roller is a simple widget that adds a d20 Dungeons & Dragons dice roller to any widgetized area of your site. In the admin you can set whether or not to allow users to enter their skill or combat bonus modifier to the roll. When selected the users can enter the modifier before rolling.
Clicking roll will roll a 20-sided die and display the modified result, as well as the natural d20 roll, so you can determine if the roll was a crit based on your crit threat range (or, shudder to think, if it was a natural 1).
The D&D Dice Roller is a simple and effective d20 rolling widget. You can see it in action at the Awesome Dice blog at
http://www.awesomedice.com/blog/ as well as dice rolling widgets for other RPGs, including World of Darkness and a generic RPG dice rolling widget