Symbian Series 60

Symbian Series 60 phones:

Nokia 6630
Nokia 6260
Nokia 7610
Sendo X
Nokia 6600/6620
Siemens SX1
Samsung D710
Panasonic X700
Nokia 7650
Nokia 3650/3660
Nokia N-Gage QD
Nokia N-Gage

The 60 series in particular offers a slew of advanced features.
It can play stereo audio and advanced games that use its bluetooth and messaging infrastructure.  Unfortunately, this increased functionality has given way to malicious code.  Two major viruses have already been found traveling from phone to phone.

Related articles:

A Bigger, Better, Cell Phone Virus
Bluetooth Phone Virus Hits U.S.