Metal Slug is unique in the world of gaming as being a hardcore run-and-gun shooter which practically everyone has heard of.
Despite the fact that it launched on the super-exclusive Neo Geo system in the mid '90s when the world was going giddy over 3D visuals, the first title in the franchise found a solid fanbase and has since gone on to sire several sequels across a wide range of formats. In addition to this, past titles have been lovingly repackaged for digital download and physical bundles, like the Metal Slug Anthology for the Wii.
You might not realise it, but Metal Slug has quite a complex history, and Daniel Ibbertson of Slope's Game Room fame has created one of his typically brilliant and very in-depth videos to tell the tale. The feature includes unique Nintendo-focused offerings such as Metal Slug Advance (GBA) and Metal Slug 7 (DS) as well as every other major entry.
If you're a fan of the franchise or merely want to get up-to-speed, this is well worth a watch.