The game itself only lists the arrow keys and space as possible inputs, but it turns out there are more:

•   Up / W / Space          - Jumps
•   Down / S                    - Climbs down a step
•   Left / A                       - Goes left
•   Right / D                    - Goes right
•   R                                - Resets game and returns to title screen
•   - / +                            - Decreases / Increases the volume
•   0                                - Toggles mute

Title screen can be exited by pressing any key, not necessarily space.