Green Man Gaming are charging a meagre £3.49 for Quantum Conundrum, the dimension-hopping puzzler from Portal co-creator Kim Swift. The campaign delivers a solid dose of thought-provoking puzzles, though it's occasionally let down by some inconsistent platforming elements.
What makes GMG's deal especially tempting is the inclusion of the Season Pass, which nets you two extra DLC packs (this edition would usually run you £13.99). It registers on Steam. Update: credit to dmorison at HotUKDeals, who found this before we did.