TrueCrypt is a free/open source disk encryption system for Windows. It creates encrypted volumes on a hard disk and allows users to mount these volumes as drives in Windows using a secure encryption algorithm, passkey and optionally keyfiles for two-pass authentication.

Mounting a volume using the command line

truecrypt.exe /v C:\VolumeName.example /lC /a /q

Note: use the parameters /l + drive letter e.g. C:\ = /lC

Dismounting a volume using the command line

truecrypt.exe /q /dC

Note: use the parameters /d + drive letter e.g. C:\ = /dC

Also see the TrueCrypt Documentation