Has a diagram of IPA symbols; you can click on a character and any diacritics and it appears in a text box. You can then copy and paste from the text box into a document that you're composing.
The following interactive charts of the International Phonetic Alphabet (IPA) were designed by Eric Armstrong of York University, Toronto, Canada; and voiced by Paul Meier, of the University of Kansas, USA. They are provided as an aid to students of dialects and phonetics. Vowels, consonants, ingressives, suprasegmentals, intonation, diacritics, ejectives, implosives, diphthongs, and clicks are demonstrated. Clicking one of the charts below will link you to a Flash animation, which will play the sound for each symbol.
IPA Writer is a free online tool to transcribe phonetics using the IPA. It is powerful, yet simple. Clicking on an IPA character adds it to the end of the "IPA result" textbox at the top of the page. The IPA Writer is separated into four parts. Vowels, consonants, other consonants and diacritics/suprasegmentals.
In Windows, the Polytonic Greek support is a combination of a special Greek keyboard layout (Greek Polytonic) and a few fonts containing all the additional characters (diacritical and punctuation marks). This downloadable whitepaper outlines the steps you need to take to use Polytonic Greek from fonts to necessary keyboard layout.