Mike's Brewing Calculators:

ABV #1
Enter original and final gravities, get alcohol content and attenuation.

ABV #2
Enter original gravity and yeast used, get attenuation range, target gravity range, and alcohol content range.

IBU Calculator
Enter your boil gravity and amounts, alpha acid percentages, and boil times for up to six hop additions and get total IBUs (based on Tinseth utilization figures).

Specific Gravity / Plato
Convert Specific Gravity (i.e. 1.044) to degrees Plato (i.e. 14) and vice versa.