OK I know Google Voice just came out but this is another stellar alternative if Google isn't your cup of tea.
Try it out..its free.
Vlingo turns your words into action.
NEW: SuperDialer provides one-click calling for an infinite address book, and unlocks reviews, maps and directions for any business.
Speak to send txts & emails, update Twitter, dial & more. Vlingo even speaks incoming txts & emails while you drive.
Update: Fixes for 2.2/Froyo
Latest version: 1.5.2 (for Android version 2.1 and higher)