mongodb.js

// Old style mongodb group and a bit more of javascript to complete the task (Array.sort and Array.slice)
db.Session.group({
  key: {user: true}, 
  cond: {
    serviceId: '888', 
    end: {$gte: ISODate('2012-08-30T00:00:00Z'), $lte: ISODate('2012-08-31T00:00:00Z')}, 
    start: {$lte: ISODate('2012-08-31T00:00:00Z')}
  }, 
  initial: {durationSeconds: 0}, 
  reduce: function(obj, prev) {prev.durationSeconds += obj.durationSeconds;}
}).sort(function(a, b) { return b.durationSeconds - a.durationSeconds}).slice(10, 15)

// Using the new (mongo 2.2) aggregation framework. Pure server side, no need of extra JS postprocessing.
db.Session.aggregate(
  {
    $match: {
      serviceId: '888', 
      end: {$gte: ISODate('2012-08-30T00:00:00Z'), $lte: ISODate('2012-08-31T00:00:00Z')}, 
      start: {$lte: ISODate('2012-08-31T00:00:00Z')}
    }
  },
  {
    $project: {
      user: 1,
      durationSeconds: 1
    }
  },
  {
    $group: {
      _id: '$user',
      durationSeconds: {$sum: '$durationSeconds'}
    }
  },
  {
    $sort: {
      durationSeconds: -1
    }
  },
  {
    $skip: 10
  },
  {
    $limit: 5
  }
)