It would be pretty easy to write one yourself. Database with 3 columns, site id, number of points, number of votes. Rating is round( number of points / number of votes )
Well say you have a rating scale of 1 - 5 : Every time someone votes, the scale number they click is added to the cell that contains the points, and the number of votes is updated by + 1. Then you divide the number of points by the number of votes. For example, you have a site that gets a rating of 4 and then 5. The number in the points cell would be 9 and the number of votes would be two. After a simple division statement you would get a rating of 4.5. I also wrote a script like this for my tutorials site. But mine used CSS hovers and what not for stars and also rounded answers for numbers without decimals.