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This problem was solved 80 years ago. (See here for XKCD-author Randall's explanation).

Of course, this answer is late, so it will never get upvoted :)

[Edit] Here is some code:

///<summary>
///Returns a rating for the given post.  Larger is better.
///Based on the equation found at http://www.evanmiller.org/how-not-to-sort-by-average-rating.html
///</summary>
public double GetPostRating(int numPositiveVotes, int numNegativeVotes)
{
    int totalVotes = numPositiveVotes + numNegativeVotes;
    if(totalVotes == 0)
        return 0;
    
    const double z = 1.96; //Constant used for 95% confidence interval in a p-distribution
    double positiveRatio = ((double)numPositiveVotes)/totalVotes;
    
    //Crazy equation to find the "Lower bound of Wilson score confidence interval for a Bernoulli parameter"
    //Again, see the above webpage
    return (positiveRatio + z*z/(2*totalVotes) - z * Math.sqrt((positiveRatio*(1-positiveRatio)+z*z/(4*totalVotes))/totalVotes))/(1+z*z/totalVotes);
}

Here is an example of the kind of ratings it returns.

IMO, downvotes should be weighted more than upvotes (controversial answers should be at bottom), and newer answers should be given preferential treatment. But even without those, this is a neat improvement.