http://www.google.com/transit

Nowadays any respectable online map service provides driving directions, but Google Labs is now testing transit directions. It is currently only available for Portland, Oregon and with only most basic functionality, but seems to work well enough:

• TriMet bus as well as rail lines
• time-aware scheduling (the samples specify routes "at 7pm on 12/9" or "by 8pm") as well as ride and walking duration. Times around the time are provided and clickable.
• total fare, with a comparison to the cost of driving (based on the IRS mileage allowance)
• maps of the route and the neighborhoods of end points and transfers.

Not (yet) available:

• restricting by mode (e.g. bus-only, commuter rail-only, subway-only etc.)
• weighting a search to minimize time, cost, transfers, or walking
• searching by landmarks (general weakness of Google Local) including station names
• accounting for construction or other delays/closures (not that we expect this from driving directions of course)
• a list of available stops or lines from which to select start or end points
• displaying all stops enroute
• links to the relevant operator/transit authority
• reflection of special fares such as senior or student-- but maybe
• disabled accessibility information

At the least this should become handy for traveling, when you might not even know the name of the local agency.