As mentioned in omega's w/u there's a more comprehensive form of the algorithm for calculating Easter dates. The method below has been given by Spencer Jones in his book General Astronomy. Unlike the formula given by Gauss, this method has no exception and is valid for all years in the Gregorian calendar, that is from the year 1583 on. The procedure for determining the date of Easter is as follows:
Divide by Quotient Remainder
the year x 19 - a
the year x 100 b c
b 4 d e
b + 8 25 f -
b - f + 1 3 g -
19a + b - d - g + 15 30 - h
c 4 i k
32 + 2e + 2i - h - k 7 - l
a + 11h + 22l 451 m -
h + l - 7m + 114 31 n p
Then n = number of the month (3 = March, 4 = April), p + 1 = day of that month upon which Easter Sunday falls.
If you want to use this formula in a computer program, the calculation of the remainder of a division must be programmed carefully. Suppose that the remainder of the division of 34 by 30 should be found. For instance we find
34/30 = 1.133 333 333
the fractional part of which is 0.133 333 333. When multiplied by 30, this gives 3.999 999 990. This result differs from 4, the correct value, and may give a wrong date for Easter at the end of the calculation. So it's necessary to use some programming trick (a rounding of some sort on the remainder should work fine in most cases I guess) to avoid this problem.