Choke is an Enchantment that costs 2G (2 colorless, 1 Green) to cast. It prevents Islands from untapping during their controllers' untap steps. Not only does Choke shut down basic Islands, but also any of the Ravnica shock lands that count as Islands (Breeding Pool, Hallowed Fountain, Steam Vents, and Watery Grave). In Legacy, it also includes the original dual lands: Tropical Island, Tundra, Underground Sea, and Volcanic Island.
Starting in December 2014, Choke shot up in price from about $1.50 a copy on average to nearly $6 in March 2015. With many Modern Green decks sideboarding at least 2 copies, Choke's value has skyrocketed. With how well the Abzan (Green/White/Black) version of the Rock has done in Modern, Choke has been the perfect way to deal with control players. Essentially, Choke will paralyze opponents in those matchups. It's also seeing some play in Legacy, as Islands rule that format.
If you happen to find copies of Choke sitting around in old uncommons boxes at your local game shop or in your bulk boxes at home, keep in mind that it's roughly a $5 card nowadays. It's definitely one for the trade binder.
(UPDATE JULY 2015: Choke is now over $8 a copy. WOW! Definitely still a card to find!)