I preface this little article by saying that you could use anything that you can easily update a feed with, not just Twitter.
1. Firstly, set up Twitter to send notifications to your email account.
Twitter doesn't do this natively, so you will have to use your Twitter RSS feed with something like rssfwd.com - this site will send any feed to your email as they come through with a small delay.
2. Using Gmail, or a similar email client, go to set up a new filter.
For any given contact you would want to be able to email via Twitter, add a code to the filter's subject field. For instance, if you want to send Craig an email, add something like tocraig in the Subject field.
3. Then in the Forward it to box, add Craig's email address while checking things like Skip the inbox etc.
Now any time you write a Twitter update with tocraig in it, that update will be forwarded - via rssfwd.com and your email account, to his email. It won't look great, with rssfwd's footer at the base and your Twitter username
Complicated? I guess. But if you just want to send quick updates to the same people [and they don't use any of that Twitter/Pownce/Jaiku stuff] this isn't such a bad system to put in place.
Even Better: Couple this with the Twitterbar Firefox extension and you can send emails through your address bar!
