Your favorite algorithm!

Dear /prog/,
As I know that all of you are [b][u]EXPERT PROGRIDERS[/b][/u], I'd like to know what your personally favorite algorithms are.
I'd pick Hirschberg's algorithm, although Needleman-Wunsch is fine but requires O(mn) instead of O(min{m,n})^(Hirschberg).

So, what are your favorite algorithms?

/* 6.
 */
int main(void)
{
    ;
    main();
}