A toy line that was launched in 1984, robotic humanoid characters that could transform into vehicles and other objects such as semis or boomboxes. Their slogan was "more than meets the eye".
A TV show was launched the same year. The show featured pretty nifty writing for its time. Revolved around battling robot races from the planet Cybertron, the Autobots (good guys) and Decepticons (bad guys). They eventually crash-landed on earth and continued their battle for objects called "Energon Cubes", which were the power source for all transformers. The Autobots, led by Optimus Prime, were aided by human teenager Spike and his father Sparkplug (yep, those were their names) in their many battles against the Decepticons, led by Megatron. Your basic classic story of good vs. evil.
Also spawned an animated feature in the late 80s, featuring the death of Optimus Prime, and his resurrection in season 3.
Spawned various spin-offs such as "Beast Wars", "Transformers Armada", and "Transformers: Cybertron". But the best is the original.
A live-action feature film is coming in July 2007, under the direction of Michael Bay, which, depending on your taste in movies, could go either way.

A big budget ($150 Million) summer action film based upon a 1980's animated TV show based on a toy line from Hasbro that was released in 2007. Directed by Michael Bay and Produced by Steven Spielberg. It has just about the perfect blend of action and story. Sure it has lame, cliched dialogue and some bad acting but it also has Megan Fox to act as a counter-balance to distract teenage boys and the male audience in general.

This film centers around a race of friendly alien robots called the Autobots, led by Optimus Prime fighting an age old war between their archenemies, the Decepticons, led by Megatron. In search of the AllSpark, the war is brought to the planet Earth where Sam Witwicky, played by Shia LeBeouf, must intervene to aid in the Autobots quest and save himself, his family and the whole human race.

What's most remarkable about this film is the detail that went into the CGI robots (Nominated for an Academy Award although it didn't win) and the solid foundation that it paved for future Transformer films.

Over the course of it's Theatrical Run, it grossed over $319 million in the U.S. (Domestically) and $389 Million overseas (Foreign) for a Worldwide Box office draw of $708 Million. Despite the overwhelming commercial success, it was a critical bomb earning an overall 57% "Rotten" rating on Rotten Tomatoes and an overall 61% on Metacritic. The latter rating actually equates to generally favorable reviews on their review aggregator. It was succeeded in 2009 by Transformers: Revenge of the Fallen.