How can I send SMS messages from my Google Voice number without using the Voice app? I would like to send text messages via Google Voice, as it is the number I give out for voice calls. However, I find sending SMS through the Voice app to be a bit cumbersome. The new widgets improve this somewhat, but it is still awkward.

I would prefer something with an interface more like Handcent or ChompSMS.

Are there any other apps that support sending SMS via Google Voice?
This isn't the cleanest solution, but it's an option:

If you set up your google voice number so SMS messages are forwarded to your phone, every time somebody texts you, you will receive that text from a 406 area code phone number.  If you send  a message back to that number, it will reach your that person, and appear to them as if it came from your google voice number.  So you can save the 406 number for each contact as a secondary phone number, and set it as the default number to use for SMS.

(if you think the Google Voice app is cumbersome, you should try doing it from an iPhone ;) )