DoctorWho

British ScienceFiction? (loosely) TV series. Originally aired from 1963 to 1989 before being canceled due to low ratings. Came back for a 1996 TV movie, then was properly restored in 2005 and is still going strong (2009 gap year notwithstanding).

The show is notable for:

• The TARDIS. A machine that can go anywhere in time and space subject to the whims of the plot. Is bigger on the inside.
• Regenerations. By 1966, the Doctor was getting too old to fulfill the role, but the BBC didn't want to end the series. As a result, the show determined that the Doctor can, every so often, change his body and (somewhat) personality as a way of "cheating death". To date, there have been eleven Doctors, with the newest (Matt Smith) to start in the upcoming 2010 season.
• Flexibility. Episodes frequently vary a lot in tone and style from week to week. On a longer timescale, every single member of the regular cast and crew has changed since the beginning of the series's run.

Every episode of DoctorWho, including fan reconstructions of the 106 episodes that were thrown out by the BBC, is available on the network. If you're just trying to catch up to the series, then you really only need to see the episodes from 2005 onwards to get a good grasp of everything. However, many of the classic series's episodes are highly enjoyable and worth watching, especially if you don't mind cheap special effects.

Due to the popularity of the new series, there are a couple spinoffs. Neither are required to understand the main series.

• TorchWood - Set on Earth, it's "for mature audiences". This means there are much more gratuitous sex-allusions and violence. Your mileage will vary. Most people agree that each season was an improvement on the one before it, though. Well, for the first three. Then WTF Miracle Day.
• TheSarahJaneAdventures? - Aimed at a younger audience, but reasonably enjoyable (says MartinPyne) if you don't let that bother you too much.