Character description from package back: 

Rebuilt with parts cannibalized from other Decepticons, Megatron rises once again to lead his army.  Though the AllSpark destroyed him once, he can now feel its power coursing through his circuitry.  It calls him to seize his rightful place as ruler over the puny planet on which he was imprisoned and destroyed and from which he will launch his conquest of the universe.  Optimus Prime is all that stands between him and victory and the Autobot leader has done a poor job of hiding himself.  At long last Megatron knows exactly where his enemy is and he has power enough to destroy him.

Bibliography of significant appearances: Revenge of the Fallen, movie, comics and novels.

Don't hold your breath as we get through this, because this could take awhile!
- 1984 -  his original toy form so incredibly [email protected]$$ it is actually an illegal toy in the U.S. as an exact replica of a real firearm, the Walter P-38 handgun.  Since he was reformatted into Galvatron by Unicron in TFTM, he made shockingly few other appearances in the G1 series.
- 1987 - as a decoy
- 1990 - in Action Master form
- 1993 - Generation 2 (G2) green and purlple tank, surprisingly, it was his first vehicle form!
- 1994 - G2 air power rocket launcher
- 1995 - a very uncool " gobots" version
- 1996 - Beast Wars Alligator and the very well known purple T-rex Beast Wars Megatron
- 1997 - Machine Wars Megatron, his first jet alternate mode ever!
- 1998 - Beast Wars Transmetal Megatron, (my personal favorite!).  In the Japan only Beast Wars II series, there was Megastorm, who is close, but apparently not quite Megatron!
- 1999 - Beast Wars Transmetal 2 Megatron, Beast Wars Neo had Magmatron, also a reasonable facsimile but still not quite Megatron. 
- 2000 - Yet another close-but-not-quite was " Mechatron" Beast Riders in Beast Machines and Beast Machines (Mega class) Megatron.
- 2001 - Here's an interesting case!  His name in the show was "Savage" in dragon mode, and "Noble" in wolf mode, and the toy is called, " Beast Changer."  But really, he *is* Megatron!  Then, later that same year came the first 6-changer Megatron in R.I.D. Transformers.  Wow what a toy!
- 2002 - In the fist half of the year was a toy with limited release.  It's Beast Machines 'floating head' Megabolt Megatron, sold in R.I.D. packaging (Hey, we'll take it!)  With the beginning of what fans would later dub, "The Unicron Trilogy" came Armada Megatron.
- 2003 - amazingly, not a single toy named Megatron!
- 2004 - Energon Optimus vs. Megatron 2-pack (limited release), Energon Megatron.  In the Japan-only series "Robot Masters" there was Beast Megatron.
- 2005 - Legends Class Megatron and the very popular Cybertron Megatron (Leader Class)
- 2006 - Okay, here is when it really starts getting out of hand!  Let's start with the Takara-Tomy (Japan only) Masterpiece Megatron.  Cybertron (deluxe) Megatron then his first appearance as a gun in 22 years: Classics Megatron  and Ultimate Battle Optimus Prime vs. Megatron, Beast Wars 10th Anniversary Megatron.  Botcon 2006 pre-prehistoric alt mode Beast Wars Megatron (a recolor of Cybertron Ultra Cybertron Defense Red Alert), Titanium 3" Beast Wars Megatron, Titanium 3" Cybertron Megatron, 6" Titanium Megatron (based on GI Joe crossover comic?), let us nevermind the Attacktix version of this character for now!
- 2007 - Many toy forms relating to the Movie: Legends Class, G1 Real Heroes, Movie Real Heroes, Fusion Blast (FAB) Megatron, Robot Replicas, Robot Heroes vs. Pack, Voyager Megatron (frozen), Voyager Megatron (metallic) Megatron Unleashed, the definitive Megatron toy of the year Leader Class.  Then Titanium 3" movie Megatron, Titanium 6" War Within Megatron.
- 2008 - TF: Animated Activators Megatron, Voyager Megatron Cybertron Mode, 2-pack Deluxe, Leader Class Animated Megatron, Botcon Exclusive Shattered Glass Megatron, Transformers (live) Movie Fast Action Battlers Night Attack Megatron,  Premium Movie Megatron, Classics/Universe Legends G2 Megatron, C/U Robot Heroes Rattrap vs. Megatron, C/U Robot Heroes Silverbolt vs. Transmetal Megatron. Whew!
- 2009 - Animated Leader Class Shadow Blade Megatron, ROTF (Revenge of the Fallen) Legends Megatron, ROTF Robot Heroes Autobot Skids vs. Megatron, ROTF Fast Action Battlers Cannon Blast Megatron, ROTF Robot Replicas Megatron, ROTF Voyager Class Megatron, ROTF Leader Class Megatron, Classics Univers Optimus Prime vs. Megatron, Hasbro Toy Shop (HTS) exclusive Megatron.

...undoubtedly, there will be many, many more to come!  Now, I'll put out the challenge!  e-mail me if you can think of any Megatron toys that we have missed!