I find myself pronouncing a lot of English words differently from what my friends do. One of the reasons for this is the fact that English just happens to be one of the 5 languages that I speak.
A couple of these word pronunciations also lead to a healthy debate with neither party accepting defeat and the only way to conclude such a debate is to validate the pronunciation.
Not to boast, but most of these debates are won by me and there is no denying that I have lost a couple too ! Surprisingly there have been debates where both parties have lost π
When you are also in a similar situation below are some of the tools that can help you to validate the English Word pronunciations:
AT&T Natural Voices Research Version
AT&T Natural Voices is a commercial TTS engine which has support for multiple languages and is marketed by Wizzard Software.
However they do have a Research Version which is also available for general public here:
http://www.research.att.com/~ttsweb/tts/demo.php
You can select from a list of voices (accent) and even have the option to download the voice.

The service is however slow and its probably due to the fact that the engine generates audio files on the fly !
However note that you can give input more than one word (preferably separated by comma) and the engine will generated the audio file with the pronunciation for all these words.
Dictionary.com/Thefreedictionary.com
Unlike Natural Voices, Dictionary.com and FreeDictionary.com allow you to hear pronunciations for words which are there in the dictionary.

Also you don’t have the option to input multiple words. However the pronunciations are pre-compiled and hence play much faster compared to Natural Voices.
If by any chance, you don’t have a sound card, or if the drivers of your card are corrupted, these 2 services provide a pronunciation key table which could be used to decode the word pronunciation.
HowJSay.com
HowJSay is another portal similar to the Dictionary portal mentioned above where you can type in the word and listen to the pronunciation.

However HowJSay additionally lists a set of words based on the input word and I found this feature quite useful.
Windows Text To Speech Engine
Windows comes with a built-in Text To Speech engine and mainly used for the Narrator Application.
On Windows Vista, you can access the Speech Properties dialog via
Control Panel -> Ease of Access -> Speech Recognition Options
On the “Text To Speech” dialog, you can input the word and select “Preview Voice” to listen to the word pronunciation.

The same option is also available on Windows XP. (Control Panel ->Speech)
I find this the quickest way of checking on a word and does not require an active Internet connection.
For Linux users, there is an option in the form of Festival ! I haven’t tried it though.
If you know of any other service, please feel free to suggest the same using the comment form.