There is a sea of crappy flash games on the web, but every now and then, one causes you to stand up and shout "THANK YOU GOD!" upon discovering it. The game I found? Abobo's Big Adventure.
Sweet mercy this is an incredible game. You control Abobo, who is the iconic boss from the Double Dragon series, and embark on a journey to save his son. The first stage is the first stage from Double Dragon, only you control Abobo and fight a wide variety of NES characters, including Goombas, Donkey Kong, and thugs from Kung Fu.
From then, it gets nutty: one level has you swimming through level 2-1 of Super Mario Bros. while eating enemies a la Yoshi, while another puts you in a Zelda-style game.
It truly is a letter of love to any NES fan, and I had such a blast playing this last night. Parental guidance suggested: it does contain some gross humor and gratuitous violence (eating mermaids can be gruesome), so exercise caution when playing this in front of your children.
Abobo. You are cherished.