I wouldn't declare this a rip-off, per se, however, the similarities between Mega Man Battle Network and a new browser-based Lego game are quite striking...
Above, screens from Lego's Ninjago Spinjitzu Smash!. Not only is the game visually reminiscent of Battle Network's battle system, but it plays similarly, too.
Your character is limited to movement on a 3x3 grid, unable to cross over to you opponent's side. When the card gauge is full, you're prompted to select from a series of Spinjitzu battle cards -- special attacks used to deal damage to your opponent. Further, you're also given the ability to throw kunai with the space bar, the equivalent of the Mega Buster. Battles are won when your opponent's HP depletes to zero, a number indicated just below their feet.
Words can only describe so much. You have to experience the game yourself: click here to play Ninjago Spinjitzu Smash. A directory of Spinjitzu cards can be found here, which, as you'll soon discover, contain abilities comparable to MMBN's own Battle Cards.
Pretty much everything right down to the battle mechanics are "inspired" by Mega Man Battle Network. For the uninitiated, here's a few Battle Network screens for the sake of comparison:
Whoever designed the game was most certainly a fan of MMBN.
UPDATE: In an interesting turn of events, actual Battle Network graphic assets were discovered inside the game's source data...
Um, wow. Talk about unsettling.
Source: Ninjago Spinjitzu Smash! (Thanks, MaxRock)