Recent reviews by loot goblin

Rather than do a full-blown writeup on this game and how fantastic it is, which is done by most every other review and contributes nothing that hasn't already been said, I want to quickly mention how well this game executes the free-to-play model.
There is truly no need to pay any kind of money to have fun and progress in Warframe. The only thing that can be purchased with real money is Platinum, a currency separate from those you can earn through normal means. Platinum only comes into the game through people who DO pay for it, but is still easily obtainable by any player by means of trading.

Platinum, like many things in Warframe, can be exchanged between players for Mods, Items, Blueprints, and most everything else someone you could really want. High volume traded items can go for anywhere between a few cents to upwards of $20 USD in Platinum for a single, obtainable item. If you do Relic missions, for example, you're guaranteed an item worth at least 2 platinum that could potentially be up to ~25 platinum. If you hold onto the rarer items until the Relics they are obtained from go out of rotation, they could end up worth 50 platinum. These come from missions that take an average of 5 minutes or so to run. Depending on your efficiency and getting fairly lucky, you could be making about $5-$7 USD worth of platinum an hour just by playing the game and trading away the majority of your drops. The best part is that Relics are opened up to players fairly early into the game; while having hundreds of hours of Warframe helps with understanding game mechanics and how to efficiently obtain items, it's not necessary to start making premium currency without paying a penny. If you ever need the premium currency of Warframe, grinding it out tends to not be as lengthy of a process as any other game I've ever played.

Oh, and you get to be a space ninja. That's cool too.