There are a number of free riddle solvers and riddle sites online, including riddles.com, riddles-online.com, and iRiddler.com. Most riddle solving sites also contain riddles to solve.
Riddles cannot be directly entered into a grid for computer solving like a Sudoku puzzle; however, a number of sites provide help solving riddles or give solutions to the riddles on their sites.
IRiddler has a wide variety of riddles and other puzzles on its site. It breaks down riddles into either word-based or mathematics-based riddles.
Riddles.com has riddles for both adults and children. There is a button by each riddle to push to show the puzzle's solution. Riddles-online.com has answers for its riddles, which are broken into sections for easy and hard riddles, as well as other categories such as logic riddles and animal riddles.