What is the highest alcohol content achievable through brewing? What is the highest level of alcohol you can achieve when brewing beer naturally, without adding alcohol?

I know that many strong, popular beers (~10% alcohol) are simply mixed with alcohol, but what level of alcohol can be achieved in natural brewing process?
That depends on what you mean by "naturally".  There are some strains, such as the "Super High Gravity Ale Yeast" by WyLabs, which can handle up to 25%.  But there are techniques, such as freezing the beer to create a more concentrated product, which have been used to get up to 60+%.  In this case, alcohol is not added, but rather, water is removed, altering your ABV.