TinyCal is a menubar calendar with Google Calendar integration. It can show multiple months, use custom calendars, and show holidays and personal calendars. The basic version of TinyCal is free. You can use it as much as you like with no time limitations or obligations. Advanced functionality is available for $10. The full version offers the following features:
BRENTRN Not very functional on a Dual G5 PPC desktop with 10.5. Crashes when advancing month or when trying to change display from one month to two months. (Version 1.3)