There's a plethora of sites offering to aid the user in solving standard crossword puzzles, but if you've moved onto cryptic crosswords then the pickings are much scarcer. Even after you've read a good guide to the topic, you might still find it difficult to get a toe-hold into the seemingly elitist world of the cryptic setters and solvers.
If you can't find a person to lead you by the hand, explaining puzzles in detail when you ask, then you might find some computer tools of equivalent benefit. Crossword Maestro is Windows software that actually solves cryptic clues, and gives an explanation too. It's been well-reviewed, but it's not free. An online site claiming to solve crypic clues is Crossword Tools, which requires a monthly subscription. However, you came here looking for free online tools to help you, and neither fulfills that role. We have to be a bit less ambitious.
One step down from these cryptic solvers, and so less like cheating, are the many sites that return anagrams and complete words with gaps. A prime example of this is WordFun.
I previously liked the Internet Anagram Server, but by default it returns too many irrelevant answers. To improve this you need to click advanced, and set maximum number of words in each anagram to 1.
Potentially more useful, is Richard's Crossword Tools, an interesting collection of tools, including the quite useful give me a word that means both X and Y.
Finally, a more comprehensive (or at least, more commercial) version of Richard's site is OneLook. For example, if you know it's 6 letters, starting with 'c', meaning barrel maker, then enter c?????:barrel maker. It has a huge word list which includes lots of wacky words, so you'll want to select Common words only.
Naturally, using any of these tools is cheating. Don't expect to win a puzzle championship using your internet-enabled laptop. However, with practice, you should be able to solve puzzles using only what's in your own noggin.
Update 20 Jan 2006: I now heartily recommend my own Instant Crossword Tools as a pleasantly easy way to get assistance with anagrams and word completions.