Simon is an electronic game of memory skill invented by Ralph H. Baer and Howard J. Morrison, with the software programming being done by Lenny Cope and manufactured and distributed by Milton Bradley (MB). Simon was launched in 1978 at Studio 54 in New York among many 70's celebrities and a lot of disco music. It became a pop culture symbol of the 1980s.
It was the must-have item for the Christmas season in 1978, and naturally was impossible to find. To celebrate it's 25th anniversay in 2003, Hasbro assembled 14 commemorative packs including the modern Simon2, and handheld Simon (with a similar design to Simon2), a T-shirt and a tote-bag. They sold these on eBay from June 19th - June 29 2003, and all procedes were donated to charity (Boundless Playgrounds). They also sold several Simons autographed by celebrity Simon's like Neil Simon and Simon Cowell (from American Idol) and others.
Simon has been an ongoing product for MB/Hasbro since it's debut in 1978, and can still be purchased at Toys R Us and other toy stores (though the versions they sell now are a little different in design). They've also made several different versions of the Simon game over the years including the 2-player Super Simon, handheld Simon and even a keychain Simon. The newest incarnation is Simon2 that you can actually flip over for a 2-player head-to-head style game. Replace the word Simon with Senso and you have the German versions.
What's it worth? Take a look at this Mb Simon price guide: sold listings for a value indication.