data.table: Extension of data.frame for fast indexing, fast ordered joins, fast assignment, fast grouping and list columns

http://cran.r-project.org/web/packages/data.table/index.html

Enhanced data.frame. Fast indexing, fast ordered joins, fast assignment by reference, fast grouping and list columns in a short and flexible syntax. i and j may be expressions of column names directly, for faster development. Example: X[Y] is a fast join for large data.

http://datatable.r-forge.r-project.org/