Telltale Games' Christmas sale has almost all company's adventure series on sale at $9.99 (the only exception being the just released Jurassic Park). Picking up any download will also entitle you to a copy of the DVD Special Edition for which you'll pay only the postage. Series on sale include three seasons of Sam & Max, Monkey Island, and the company's adaptations of Back to the Future, Wallace & Gromit and Bone.
There's also free gifts as is fitting for this time of the year. The first episodes of Back to the Future, Sam & Max: The Devil's Playhouse and the seasonally-appropriate Ice Station Santa, the first episode of Sam & Max: Beyond Time and Space. Just be aware that (SPOILER) the latter might not be quite the feast of yuletide cheer you're expecting... Check out Telltale's offerings at the company's sale page.