Mac: We previously highlighted Fantastical as a pretty nice Mac OS X menubar calendar with too high a price tag. If you like the idea and want a free alternative, check out Calendar.
Calendar always exists in month view (and you can move from month to month like you would in iCal) but when you roll over a day you get a side panel that shows you what you have scheduled. Clicking on a day will open it in iCal so you can make changes, view the event in more detail, or do whatever else you'd want to do with it in iCal. That's pretty much all it does, but it's handy and free.