Many phone/handheld device manufacturers have licensed ActiveSync technology to provide Push mail functionality on their devices.  Here is an article which enlists which device manufaturer and OS support which feature and Comparison Table of Exchange ActiveSync Clients.

http://en.wikipedia.org/wiki/Comparison_of_Exchange_ActiveSync_Clients

Related Links:

Outlook Web App

Hosted BlackBerry

Hosted BES

Dedicated Exchange Server

Managed Exchange Server