You might have a ColecoVision in your closet somewhere, but you also don't really want to go to the hassle of digging it out and then trying to figure out how you're going to hook it up to your HDTV.  You might also have a Game Gear, but don't really want to go scrounging for 6 AA batteries.  Retro gaming is fun, but it can also be a hassle.  Thankfully, Archive.org has over 900 console games you can play in browser right now.  The titles range from classic systems such as the Sega Genesis, ColecoVision, Atari 2600, down to the joyously obscure Mega Duck WG-108 (a.k.a. "Cougar Boy").

If you feel like getting nostalgic this morning and obliterating your productivity, click over to Archive.org.  Note that if you have a USB gamepad lying around, it will make playing the games substantially better.