The popular ATmega48 series offers low-voltage operation, with excellent features!
The AVR series of microcontrollers are right up there with Microchip PIC products. Powerful, inexpensive, and programmable with some very good open source software (look up GCC), with a vibrant user community.
From the aforementioned website AVRFreaks:
The mega48 is a 4k variant of the mega88, which was allegedly designed to fill the need for an 1.8V mega8-like part. Its siblings are the mega88 and mega168; 8- and 16k variants of the same. The ATmega48, ATmega88 and ATmega168 differ only in memory sizes, boot loader support, and interrupt vector sizes. In ATmega48, there is no Read-While-Write support, and no separate Boot Loader Section. The SPM instruction can be executed from the entire Flash.
Here are more key features: