Sisters of Battle. They've been something 40k players have been waiting for. Now, you can head over to the Games Workshop website and... well, they're sold out there right now... but you can still maybe pick it up at your LGS. They've got a whole army set, as well as starter sets for some other factions, if you're so inclined.
From the website:
The Adepta Sororitas are the military wing of the Ecclesiarchy. Known by many as the Sisters of Battle, they are an army of pious and highly skilled warriors who purge heretics and enemies alike with their 'holy trinity' of bolter, flamer and melta.
This Sisters of Battle Army Set provides you with everything you'll need to start an Adepta Sororitas collection and field it on the battlefield. In addition to a 25-model Sisters of Battle army led by a Canoness, you'll receive a copy of Codex: Adepta Sororitas, as well as a handy datacards set and Warhammer 40,000 Core Rules booklet. Finally, there is a set of Miracle Dice, designed for when you're performing Acts of Faith in the game!
The codex included in the set comes with exclusive cover art and is the first opportunity to get the fully updated Adepta Sororitas rules and background – including datasheets and bestiaries for other units that aren't included in the Army Set, such as Immolators, Exorcists and Retributors.