//equivalent of MySQL "SELECT COUNT(*) AS `count`, `fieldName` FROM `someTable` GROUP BY `fieldName
db.someCollection.aggregate([{"$group" : {_id:"$fieldName", count:{$sum:1}}}]);

//as above but ordered by the count descending
db.someCollection.aggregate([{"$group" : {_id:"$fieldName", count:{$sum:1}}}, {$sort:{'count':-1}}]);