My off the radar spots:
The best and secretest is the newest but the secret is starting to get out. TANUKI on NW 21st, between Flanders and Glisan. It's an upscale Okinawan izakaya. Can't wait for them to add takoyaki and ramen to a menu that alright highlights okonomiyaki and rafutei. Yeah, it's in a white-bread part of town, but they pull no punches, and even I get blown away by that menu (and I thought I knew Japanese pub food).
Other secret spot: TAMBAYAN on SE Foster. Total home-style Filipino food. Only non-Pinoys I ever see there are the ones married to a Filipino.
A great taco truck: LA CATRINA. It's on SE 82nd just south of the Johnson Creek shopping center, parked in a gas station. Awesome awesome tortas and tacos. Their cubanita is sort of a riff on a cuban sandwich. Make sure you order the small, because the large is just really really large.
Another taco truck that I want to re-visit: LA ESTACION. It's up on Killingsworth parked next to a strip club, but they have Yucatecan style good, including panuchos and cochinita pibil.
My not-so-secret, but definitely still locals only:
NGOC HAN BUN BO HUE, on 82nd just north of Division. In addition to great Hue-style noodles, they also have really nice hu tieu and they even do lots of shakes, made of stuff like jackfruit, papaya and durian.
DANG'S THAI KITCHEN, in Lake Oswego. While Pok Pok is the sexy response for out of towners looking for Thai food (and I agree with it), sometimes if you're local you just want the classics. For me, the place that does it best is all the way south of Portland, but it's worth the drive, and in truth some dishes like the khao soi are better at Dang's than they are at Pok Pok. It's just that Pok Pok gets all the national media acclaim (and rightfully so) and so we steer tourists there looking for the best of Portland's best.
It's worth mentioning the ASIAN STATION food cart on the corner of SW 10th and Alder. I've talked about it in other threads, but they do a great xiao long bao, which won't be at the same heights as the Din Tai Fungs of the world, but it'll do great considering that it's in a small food cart in downtown Portland. (The closest competitors in terms of quality will probably be in Seattle, Vancouver, and beyond that, the Bay Area and LA.)
As you can see, Portland is quite the food town. I don't know if you were truly looking for off the radar spots or just soliciting ideas for a restaurant trip of your own.
You can do quite a bit just following ExtraMSG's tip sheet:
That tip sheet only scratches the surface, which is as he intended since he wanted people to do a lot of their own exploring. Truly, a lot of the responses offered here on Chowhound by us Portlanders tend to only scratch the surface, as most people are only visiting for a short period of time.