The Cherriots bus system has a state of the art terminal in downtown Salem, and lots of hyrid or natural gas buses. The system goes between Salem and outlying communities. I paid 75 cents for a two mile ride from Salem to Keizer and found the service good, but it doesn't run on Sundays. See the link below for more details about routes, schedules, and prices.
I get there by air, flying into Portland. A couple times we flew into Eugene.
We borrow one of my folks vehicles and double their mileage on it. Another really neat way to travel around (or through) Oregon, is on the Amtrak Coast starlight. We did that a couple times to go from Salem to Seattle and the scenary was fantastic. It's also rather cheap. I love sitting in the viewing car.