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EDIT: This game has been discounted to $2.99 at Desura! (70% off)
The side scrolling indie game Tesla: The Weatherman is 66% off at Desura, which makes it $3.99 ($9.99). Alternatively, you can purchase it straight from the developers (Thoughtquake Studios) for $3.33.
The game stars Nikola Tesla and you have to fight robots and stuff lol.
Indie Gamers gave it a good review.
A demo is available also.
EDIT: This game is also available for $3.33 at GamersGate US or £2.10 at GamersGate UK.