Valve is providing the game Portal for Free until 24 May. There's no catch - as long as you download and install Portal prior to 24 May, you can keep playing it forever for free. The only requirement is that the Steam Client be installed.
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Mind I was shooting these at 30 FPS limit (by FRAPS), and was playing with Rumblepad 2 (Gamepad). The idea was to walk through step by step.The maps are not as easy as they may look, try them.