Keeping an open line (that often moves frequently) while the game is in play.
Many books offer this on key games (e.g. Monday Night Football).
Dangerous for the book, esp. given time delays in cable and DirectTV feeds. Mitigated somewhat by more juice, a larger spread between the sides, and low limits.
I've yet to see a good software implementation. I had begun working on one in my former job.
Spreadbetting is another popular variation of in-play wagering.