The Internet has a number of useful tools and resources for developing your understanding of personality type. Remember - don't take the results of ANY questionnaire at face value, but always explore further to decide which type you think fits you best when you are not trying to please other people. (These links all work as of Dec 2013)
16 Personalities - A great little questionnaire that lets you choose a range of responses. The descriptions are reasonably through, and explore personality, relationships and work life.
Free type quiz and descriptions - Team Technology have developed some good descriptions and a number of free reports. Try taking it a few times, see if your results differ and if you can find the type that fits best.
Personality type & career - A good, thoughtful quiz by author Paul Tieger, allowing considered self-assessment with understanding. It has recently been re-worked, and now includes an optional paid-for career report.
The sixteen Myers-Briggs types - Some really excellent longer descriptions (up to about four pages) of each of the types. Draws out more of the subtle differences between whole types, to help you decide between them.
Jungian function theory - This is the basis of the M-B system, and gives a clearer idea of the dynamics involved in the inter-relationship between preferences. The 'working out the letters' approach is really just a shortcut for getting at the underlying dynamics discussed here. A good, clear description.
Neuroscience of Personality - A fascinating talk by Dario Nardi, introducing the in-depth research he has done looking at personality preferences and their roots in how our brain works. It's the science behind how the Jungian Functions map to the areas of the brain we use.
More descriptions of 16 types - A well-developed and thorough set of profiles which can also be purchased on a CD for use by practitioners in workshops and assessments. Web site includes pages of application info.
Working out your team role - A look at one's role at work and a comparison with psychological type, providing a possible basis for understanding the stress points and potential directions for change or challenge in your job.
MBTI Complete is an American site which gives you the official MBTI questionnaire online and then leads you through a self-analysis to help you find your best-fit type. The site is supportive and informative, costs $60 and may be a good back-up option if you can't fnd a qualified practitioner to help you.
British Association of Psychological Type - a UK organisation supporting people interested in psychological type, whether practitioners or not. They have an annual conference and a quarterly journal.