Extensions:

*Writer
 Application Document-  .sdw  

 Master Document-  .sgl

*Calc
.sdc

*Impress
.sdd

*Draw
.sda

*Base
.sdb

*Math
.smf

Writer: a decent word processor. Some people say it's better than MS Word.

Calc: the Open Office spreadsheet program and a replacement for MS Excel. 

Impress: a presentation program that replaces MS PowerPoint. 

Draw: a two- and three-dimensional drawing program that many people rate more highly than the same functions within MS Office.