Question: I am a product/project manager for a product we've created. We have customers using the product, which we are continually updating as it is Web based.

What are some tools and techniques for managing communications with clients regarding product updates?

Answer: 
I would suggest **to avoid** any promotional one-way communications in this particular case (and in almost every other case). Try to make the product speak for itself without a product manager behind every message. One of the methods is "[promotion through education](http://gettingreal.37signals.com/ch13_Promote_Through_Education.php)". Turn your customers into evangelists. They will promote your product.
If your updates are small and can be made public, I would include Twitter in your list. Take [the Twitter stream for OmniFocus](http://twitter.com/#!/OmniFocus), [or for FreshBooks](http://twitter.com/#!/FreshBooks) as examples.