AltSoftSerial Library

Improved software emulated serial, using hardware timers for precise signal timing and availability of CPU time for other libraries to respond to interrupts during data AltSoftSerial data transmission and reception.

http://www.pjrc.com/teensy/td_libs_AltSoftSerial.html

If you need very low baud rates, check out SlowSoftSerial by Paul Williamson.