Zamzar.com, a great free online file conversion service has recently added a very smart Text to Speech converter. With this converter you can convert TXT, Word, PDF, Word Perfect and OpenOffice document files to Speech. So this free online converter could be very helpful for you if you are looking to perform a quick Text to Speech Conversion without any special Text to Speech Converter.
There is a file size restriction on zamzar therefore any input file size should not exceed 100MB. But since you would be dealing with Text files for this Text to Speech converter therefore you should not worry about the 100MB file size limit.