Although a version called Monopoly: Here & Now: The World (sweet mercy, that title!) was previously released on iPhone, there is also now a plain ol' version of Monopoly for $5 on the App Store. The game supports up to four players on the same network or through "pass 'n play." There's also the option of a two player game through Bluetooth multiplayer.
We had previously criticized the Xbox 360 take on the classic board game for costing $30, especially when the non-virtual version is under $20 at most retailers, so $5 doesn't seem like such a bad deal.