Fortunately, the imagination of the show's creators and writers often has a bigger budget than the television medium can afford, allowing them to continue the tales of their heroes in print format. Novels, comic books, and short stories can extend the lives of your favorite shows for many "seasons". Not all shows are fortunate enough to get that extra lease on life, but that doesn't mean your own imagination can't continue the journey.
In that vein, Charlie Jane Anders suggests some awesome books to replace your favorite cancelled TV shows. From Terminator and Angel to Journeyman and Jericho, many of your favorite (but cancelled) science fiction series of the last decade are represented by equally modern literature. Even if you were satisfied with your favorite show's run and are just looking to try some new authors or series, this list is a great place to start.
For my part, I think I'll go add Robopocalypse to my to-read list, making for a nice change from my usual Star Trek pulp.
However, I draw the line at adapting my favorite books into video games.