First there was Funambol, the "open source mobile application server that provides push email, address book and calendar (PIM), data synchronization, and device management for wireless devices, leveraging standard protocols such as SyncML. For users, this means BlackBerry-like capabilities on commodity handsets." [wikipedia]
Funambol Mozilla Plugin is a Mozilla plugin that allows the synchronization of calendar(s) and addressbook between Mozilla Suite (Firefox and Thunderbird) and the Funambol server and, therefore, with any SyncML client (such as mobile phone, PDA, ...) [download]
New from emoze is a service that will "push" your personal emails to nearly any mobile handset that supports email, including Nokia, Samsung, and Motorola devices. With the emoze service, you can receive your emails as well as manage other Outlook data, like your contacts and meeting requests. In addition to Microsoft Outlook & Microsoft Exchange, emoze also supports Lotus Notes Domino servers and POP3 online email services such as Hotmail, Yahoo! and Gmail. Yes, Gmail. Push Gmail. Unbelievably, the emoze service is totally free.