WinSuk.net
Twist Chat

Type and send SMS with your wrist - using Pebble's accelerometer!
Great for short replies, and capable of long messages if desired.

Just in case, here are the controls:
Wrist rotate:Choose letter/letter group
Select:Insert letter/select group
Select long-press:Send message
Up:Backspace; return to group list
Down:Shift > Caps lock > neither

Future plans (high-to-low priority):
- Pick a contact to send to instead of a thread
- Type a number to send to instead of picking a thread
- Customizable hotwords and emoticons
- Message history (not entirely needed as Pebble has notification history)

Available on the Pebble App Store
Android app on Google Play

Complete source on GitHub