skame/data.table

R package data.table extends data.frame. Fast aggregation of large data (e.g. 100GB in RAM), fast ordered joins, fast add/modify/delete of columns by reference by group using no copies at all, cells can contain vectors, chained queries and a fast file reader (fread). However, the main benefit is its natural syntax: DT[where, select|update, by].

Overview is on the project's homepage.