TITLE: Open problems with monetary rewards
QUESTION [104 upvotes]: Since the old days, many mathematicians have been attaching monetary rewards to problems they admit are difficult. Their reasons could be to draw other mathematicians'  attention, to express their belief in the magnitude of the difficulty of the problem, to challenge others, "to elevate in the consciousness of the general public the fact that in mathematics, the frontier is still open and abounds in important unsolved problems.1", etc.
Current major instances are

The Millennium Prize Problems
Beal's conjecture

Other problems with money rewards

Kimberling's  list of problems

Question: What others are there? To put some order into the answers, let's put a threshold prize money of 100 USD. I expect there are more mathematicians who have tucked problems in their web-pages with some prizes. 
What this question does not intend to achieve:

once offered but then collected or withdrawn offers
new pledges of sums of money just here

P.S. Some may be interested in the psychological aspects of money rewards. However, to keep the question focused, I hope this topic won't be ignited here. One more, I understand that mathematicians do not work  merely for money.

REPLY [4 votes]: Domokos and Várkonyi offer $\\\$\frac{1,000,000}{F + E + V − 2}$ for the polyhedral Gömböc with the minimal number of flat faces, where $F,E,V$ are the number of faces, edges and vertices of the polyhedron.