My son attends WoodHaven School, which is a Waldorf-inspired cooperative preschool near 217 & Canyon (behind Kohl's) in Beaverton. The curriculum is play-based and environmentally focused and each class is limited to 12 students. Instead of a play structure, the kids explore outside in an amazing forested play area. The school is also completely media free, including student dress and lunch boxes, etc. As a cooperative, one parent helps in the classroom each day, and each family has a parent job. You can check out the website at woodhavenschool.com. They have open houses the second Monday of each month if you want to meet the teacher and check out the facilities.
I also visited Multnomah Playschool and thought the teacher was amazing. It's also a cooperative school though, and I wasn't able to set up an arrangement to have someone watch my younger daughter while I volunteered in the classroom. Finally, I thought Gabriel Park Preschool and Village Preschool on Terwillager were both good, but I wasn't as excited about the large class size at Village Preschool (think it was 18 students), though they have a great set up where the kids can go inside or outside at any time because the classroom has a huge wall of windows.
If you can't tell, I visted a lot of schools before deciding on WoodHaven. If you're just looking to see what's out there, the website readysetmom has a great listing of all preschools in the Portland area. In the end, I felt most comfortable with a co-op school because I wanted to be involved in his classroom. I drive a bit farther than I'd like, but I really fell for the outside play area at WoodHaven and think the teacher is wonderful. Plus I love that there are wooden toys, no media, and that the kids learn so much about nature. My son notices and points out so many things now when we're outside.