1. SMS gv extension - sends texts received on your google voice account to the messaging app, and allows you to reply via the messaging app using google voice. I have no messaging plan and just use this, saves me $30/ month
2. BiteSms - love being able to quick reply without having to leave the app I'm in. Can also use it for an instant speed dial by hitting the star in the quick compose, which will bring up your favorites list.
3. Sbsettings and weeflashlight for the notification center - being able to toggle most settings and even respring your device from the notification center is really, really convenient. Weeflashlight let's you use your flash as a flashlight by hitting a button in notification center, I know it sounds silly, but I use this ALL THE TIME. Look under the couch, drop your keys getting out of the car at night, etc.
Hope this helps!