3 Desktop's (ALP, CMIME, CICAL)
ALP - Used when looking up an address from the Global Address List (or it's counterpart in a Domino environment)
CMIME - Used for sending email. NOTE: this is required to send, and is usually the cause of not seeing a send option when composing a new mail...this is NOT required to receive email.
CICAL - Used for wireless calendar.
Other ones you may see are...
IPPP - Used for internet over MDS.
SYNC - Used with 4.0 for wireless PIM sync.
In case anyone was wondering