Solving tools
Overview
There are a variety of resources available that can help you get the answer to clues you are stuck on. These can be used for quick and general knowledge crosswords as well as cryptic puzzles.
Some people are reluctant to use these tools as it seems like cheating, but they can improve your solving skills — extending your vocabulary, general knowledge and familiarity with how words are used in cryptic clues. It would of course be unethical to use them in a competition.
In general, these resources are going to be of less value if you already have access to the answers to a puzzle and can look things up on the internet.
Crossword dictionaries
Specialist crossword dictionaries such as Chambers Pocket Crossword Dictionary provide lists of words appearing in crossword clues. Entries are followed by possible synonyms, examples if the entry is a category, and possible functions within cryptic wordplay.
Bradford's Crossword Solver's Lists is organised into categories alphabetically, with lists of possible clue answers within each category. Anne R Bradford has built this up from her own solving experience since 1957.
Word finders
These list the possible words or phrases that will fit a particular letter configuration. They are also referred to as crossword solvers.
For example, if you had already solved some clues, and so knew that the answer to 11 Across (see diagram below) needed to be a 6-letter word with the letter configuration _L_C_D, then a word finder would offer PLACED, PLACID and SLICED as possible alternatives.

Print versions of word finders have been superseded by IT alternatives, which are more versatile and comprehensive. On line solvers include:
The WordWeb Pro application also includes a word finder.
An application that solves cryptic clues
William Tunstall-Pedoe has developed a computer program for solving cryptic clues. This interesting application use Artificial Intelligence techniques to analyse the clue and then come up with an answer. A success rate of 75% on typical cryptic clues is quoted, approaching 100% if some of the letters are known. The program also offers wordplay explanations where it can.
The app is available for iPhones and iPads and is free to download.There is a subscription service to unlock special features and purchase additional material.