Here are some brainteasers I've found and enjoyed. Hopefully you'll like them as well.
I found these brainteasers on my former colleague Rustan Leino's web page. That page is no longer available, and even when it was available it didn't include solutions. So, with his permission, I put the brainteasers and their solutions here.Knight, Knave, Commoner #1 Knight, Knave, Commoner #2 The Hidden Card #1 The Hidden Card #2 The Electrician Problem Points on a Circle Stabilizing Nodes from an Anchor Table with Four Coins The Prisoners and the Switch Line of People with Hats #1 Line of People with Hats #2 Three Hat Colors Multiple Hat Colors Flipping Cards Burning Ropes to Measure Time Boris and Natasha Determining a Hidden Digit Placing Coins on a Table The Worm and the Rubber Band Right Triangle with a Given Side Summing Pairs of Numbers to Primes The Exact Batting Average Passing Alternating Numbers of Coins A Special Squarish Age Psycho Killer Mixing Vinegar and Oil Path on the Surface of the Earth Fair Soccer Championship Cutting Cheese Finding a Counterfeit Coin Children and Light Switches Subsequence of Coin Tosses Voting on Coin Distribution Determining the Number of One Hat Coins in a Line Multiples in the Fibonacci Series Mixed-up Airplane Seats Colored Balls in Boxes Catching a Spy Making a Square Larger Frugal Selection of Weights Opening Boxes in a Courtyard Finding a Restaurant in a Park Dropping 9-Terms from the Harmonic Series Poisoned Wine Lemmings on a Ledge Witches at a Coffee Shop Finding a Hermit Genders of Children Three-Person Duel Capturing a Pirate Ship Ages of Children Dropping Eggs The Duck and the Fox Guessing Each Others' Coins Weighing Piles of Coins Digit Sums of Multiples of 11 Alternating T-shirt Colors Chomp Rectifying a Pill Mistake Handshakes at a Dinner Party Car and Key Hide-and-Seek Transporting Bananas Planar Configuration of Line Segments Enclosing Land by Fence Pieces Picking the Larger of Two Cards Evaluating a Polynomial A Game of 2009 Cards Half the Area of an L