The % Alcohol Calculator will let you determine the percentage of alcohol in a beer (both by weight and volume), the attenuation levels and the Residual Extract amounts. All that is needed is the Original and Final Gravity values. (or just the Final Gravity for the refractometer/hydrometer tool, see below).

Additionally, should you have both a refractometer and a hydrometer, ProMash will let you determine these values with an even higher level of accuracy with only the FG reading (from both the refractometer and hydrometer). This tool will also let you backtrack and determine the Original Gravity should it not be known. You get to this calculator by pressing the Refractometer button on the % Alcohol Calculator: