On April 25, 2000, the Clay Mathematics Institute announced that it would give $1,000,000 to anyone who could solve any of these seven outstanding problems in mathematics:
All of these problems are classic questions that have resisted solution for many years, and the Clay Institute wishes to stimulate interest in them. There are strict rules on what constitutes a valid solution.
David Hilbert did a similar thing a century earlier in 1900, when he proposed his twenty three so-called 'Hilbert Problems' that required solution. To date twenty have been resolved. It is interesting to note that one of the three left is the Riemann Hypothesis which features above, now considered to be the most important outstanding problem in pure mathematics.
[Editor's note, 8/31/2006: Fixed link to Poincaré Conjecture and added <p> tags.]