Google Knol tests a cool feature that automatically converts articles to audio files, which can be played or downloaded. For now, the option is available for a small number of knols, like this one about multiple sclerosis.
"We are experimenting with Audio Playback as an option for some knols, starting with a handful of English language featured knols. If this experiment is successful, we may make Audio Playback available to more knols in additional languages, and additional features," mentions a Knol help page.
Hopefully, Google will add support for text-to-speech conversions to other services like Google Docs, Google Reader or Google Book Search.