FreeOTFE

Available under a free, open source licence, FreeOTFE is an "on-the-fly" (OTFE) transparent encryption program that allows one or more "virtual memory cards" to be created on any Windows Mobile PDA. Data written to such a virtual memory card is automatically encrypted before being stored.

FreeOTFE offers multiple encryption options allowing control over how the data is encrypted, and includes multiple hash/cypher algorithms (including AES, Twofish and Serpent)

A PC version is also available, allowing data encrypted on your PC to be read/written on your PDA, and vice versa.