Monster Master is a flash game that, like Mytheria, simulates a TCG in shrunken, simplified and less diverse format.
The model for this one, though, is Yu-Gi-Oh!, the TCG associated with an anime and manga that my sister is doing an extended fancomic for (and for whose Yu-Gi-Oh!-related blog I have done a couple of posts), which is based on Magic: The Gathering in very loose fashion, but has radically different rules.
Monster Master's (and by extention, Yu-Gi-Oh!'s) paradigm is quite simple. Once per turn, you can play ("summon") a Monster from your hand. In Yu-Gi-Oh!, stronger Monsters are limited by various restrictions, the simplest being requiring sacrifices; in Monster Master, it's a little bit simpler-Monsters have extended "summoning sickness" based on their strength. Dragons and Hydras take three turns to shake it off, while Rats and Leprechauns (yes, really) can attack immediately.
However, where Yu-Gi-Oh! has a very simplified "strongest wins" combat system with overflow damage, Monster Master has a sort of weighted "dice roll" combat system.
This is INSANELY frustrating, as a Monster with one defensive point can repeatedly fend off a creature with twelve offensive points, and other times, a significantly weaker critter can totally thrash a strong one attacking it. (Or whatever they're called.) A likable bit is that each Monster also has its own individual hitpoints, which is lacking in the Yu-Gi-Oh! combat system. However, Monster combat is incredibly frustrating and is like slogging through deep, squishy mud-not much fun.
What is fun are the spells. Yu-Gi-Oh!, because of the game's size and age, has a lot of diverse cards, the strongest of them being heavily restricted or banned. Monster Master has only a handful of cards, and a lot of its spells are based on cards that were too powerful to be allowed in tournament play, or sometimes even too powerful to be printed.
Which means that one can open with an insane advantage over the other. If you draw Gift, for instance, which draws two cards, there's a chance that you'll draw another Gift, and I have drawn three (the maximum possible number) on the first turn, and drawn an appropriately unhealthy number of cards with which to do unpleasant things to my hapless opponent. (Which, thanks to two-player mode, was often me. Ouch.) If you've ever played Yu-Gi-Oh!, you know that Gift's equivalent, the Pot of Greed, is generally considered one of the most powerful ever printed; it was one of the first ten cards to be banned.
Anyway, this is where most of the enjoyment of Monster Master comes from: Playing cards whose power levels make you wince. The "ground combat" is a horrible slog and not very much fun. The highest success rates I've had are with decks where I clear the path with removal cards and then call out strong monsters and use relevant cards to get them ready to attack sooner and more often. The Combine card used with a Dragon and a Knight to get a Dragon Knight (followed by a Summon to remove summoning sickness and a Charge!, which allows an extra attack) is a devastating combination, as the Dragon Knight has an enormous (for the game) twelve power-which is more than half of a player's health.
All in all, not the best game, but you might get some enjoyment out of it if you enjoy TCGs.