Some Google engineers who regularly use public transportation developed a local trip planner for transit users: Google Transit. Currently it only covers the Portland, Oregon area, but they're asking for input from other transit agencies.
Locally, I use the Denver RTD Trip Planner for unfamiliar destinations. This has worked fine for me, but I'm fairly familiar with the routes in the Denver-Boulder area and I keep local route timetables in my messenger bag.
I've used trip planners while visiting other cities also. The biggest differences between Google Transit and these other trip planners:
Google Transit shows you a map, the others don't.
Google Transit seems to be much more flexible than the others in location inputs.