UnRar is a free addon for Windows Mobile 2003 and 5.0 based Smartphones that allows you to open and extract RAR files on your Smartphone.

Main features include:

1) Extract RAR archives
2) SFX Module(WinRAR self extractor)
3) Password protected Rar archives
4) Multi-part Rar archives
5) Recovery archive

Essentially it supports most of the features that the desktop version of WinRAR offers.

There already exists a version of PocketRAR for Windows Mobile based Pocket PCs.

Download Links:
Official WinRar website (without TGetFile.dll)
UnOfficial Mirror (with TGetFile.dll included)