Riddlekeeper costs 2U and is a 1/4 Homunculus. Its ability is that whenever a player attacks you or a planeswalker you control, that creature's controller puts the top two cards of his or her library into the graveyard. This can prove to be quite a deterrent for opponents attacking, especially if you're playing a Commander deck led by commanders such as Sidisi, Brood Tyrant, The Mimeoplasm, or others that use the graveyard as a resource. As part of a greater mill strategy, it's sort of a small piece.
Still, being a Commander exclusive means that Riddlekeeper has room to grow price-wise. It could double in price if the demand for it rises. It's a card to keep an eye on, as Sidisi in particular is becoming quite a popular Commander.
(Side note: the popular niche format Tiny Leaders has some use for Riddlekeeper too. It's a format with only 50 card decks with cards with converted mana costs of only 3 or less. It's typically an aggressive format, so the 1/4 body and ability could prove a lot more powerful than in regular Commander..)