WiFi Keyboard is an Android app which allows typing on your phone using your computer's browser.
It works as an ordinary input method. You can use the terminal or type SMS... Uses WiFi for low latency input. 3G may be blocked by some carriers. USB connection works best, but requires some technical knowledge and connected cable.
Available from Google Play: https://play.google.com/store/apps/details?id=com.volosyukivan
Available from F-Droid: https://f-droid.org/repository/browse/?fdid=com.volosyukivan
There is also a related project in development which provides Windows/Mac native client implementation: https://github.com/dportabella/DesktopKeyboard2Android