There are a number of different websites that help solve crossword puzzles. Some of them search by available letters, some give answers to the clue and others use a combination of the two to give more accurate results.
Dictionary.com has a crossword solver, and either the clue or answer can be used in the search. For the answer portion, the user inputs the known letters and substitutes a question mark for the unknown letters. Crosswordsolver.org lets the user input established letters into boxes while leaving the unknown squares blank. Oneacross.com uses a combination of dictionary searches and clue analysis for reliable search results.