Bitcoin wiki got a page with link to all popular calculators around the net for today. There are two most popular - Generation Calculator which can compute how much would you get American dollars (USD) in day, week or month based on your hardware hash rate and market exchange rate of 1 BTC.
The second one will show you more information about current state of the Bitcoin network and future changes in mining difficulty. This gives you more accurate assumption on your earnings in the future, which will be lower while difficulty rises.