Super Mario Bros. Crossover is an idea that anyone who has ever had an NES has had or talked about at some point in their life - what would happen if you brought characters from one game into another? Thankfully, the guy who took ownership of this idea - at least for the original Super Mario Bros. - did an excellent job of it, creating a mashup game that is more than just a cheap gimmick for the luls and nostalgia, but actually a pretty well balanced challenge regardless of which character you use.
So, you're playing the original Super Mario Bros., fundamentally pretty much unaltered. But a bunch of popular 8-bit characters are at your disposal: Link, Mega Man, Simon Belmont, Bill Rizer, Ryu Hayabusa, Samus Aran, and Sophia III (the tank from Blaster Master.) Each has most of their main abilities, if not all; this is facilitated by converting Mario's "run" button to an attack of some sort, and there's an extra button added for a secondary ability as well. Aside from their individual powers, the normal rules of the Mario universe are in play for everyone - all characters can still jump on enemy heads for kills, bump and break blocks with their heads, and power up their attacks by eating mushrooms (as well as become invincible with stars.)
You might immediately think that certain characters would become broken, walking death machines, simply stomping through levels blowing everything to pieces, such as Bill with his fully-powered Spreadshot ...
... but each character actually has their strengths and weaknesses, and it's very tough to actually complete the game with just one. Bill does stomp through early levels that aren't jump-heavy, and underwater levels, but his long, slow, floaty jumps are too fiddly to land in levels that require a lot of quick and precision jumping. Ryu's at a distinct advantage when there's a lot of stuff to grab onto, but underwater he struggles mightily. Mega Man's firepower is offset by having the worst jumping ability in the game, having to call down Rush Coil with his secondary ability in parts of many levels, which leaves him open to attack -- it's murder getting him through Lakitu levels. And none of the characters can dispatch an enemy in one hit with their weapons, requiring anywhere from two to four hits to take out even a lowly Goomba.
As it turns out, Mario Mario (who is also playable) is still the best for taking on the whole array of dangers in his world. Mario is also still the only character that can run, with all the others moving at only half his running speed at best, and since the levels were designed to largely be run through, it can be a battle with the clock for all the other characters as well.
There's impressive work in the little details -- each character has music and sound effects from their respective games, and it's impressive how they feel relatively natural and how their soundtracks still work so well when airlifted into the Mario universe. The game also has a lot of options - you can save to a local file during any level, and there's various modes of play allowing you to switch characters between levels, be locked into one character for the whole game, or play a "survival" mode where a character becomes unavailable once they die. Certain in-game achievements also unlock cheats that can be activated while you play.
But this is running too long already for something that you can play for free just by clicking a link. It's clearly a labor of love and every little detail has been worked on carefully. That, and just the pure joy and fun of crawling on the walls around Bowser's level or wiping Bloopers out with a spreadshot, is what earns this game a 5/5. Congratulations good sir, and may Nintendo not cease-and-desist you into oblivion.
* Play it
* Interesting interview with creator
* SMB Leftovers (hilarious)