Now to answer your questions as best as I can! I think it really depends on how steep your slope is, as well as just how tall your trees are and how close they are to the area you're planning on growing. My property is a north-facing slope (some areas more steep than others), with trees in all directions. When I joined permies, I asked the same question as you: https://permies.com/t/33637/cascadia/Steep-North-Facing-Slope. I haven't really tried doing to much on the steeper portion of my land, but I've noticed that in the summer I do get quite a good bit of light as the sun is higher in the horizon. In the summer, the sun rises above the trees at about 8:00am, and sets at about 6:00pm, and this gives me a good 12 hours of sun during. During the winter, however, the sun doesn't get above the trees until 10, and sets behind them at around 2:00. At most, I get four hours of sun.
It's important to try to find where your sunniest areas are, and maximize those for things like fruit trees and vegetables that require more sun. A tall hugel can also make a difference in raising the veggies up above some shadows to get more sun longer. It also helps warms thing up sooner.
As for things growing on my slope, I've had a lot of success with peas, daikon radishes, blackberries, thimbleberries, raspberries, blueberries, wild strawberries, and huckleberries. Elderberries, lingonberries, and serviceberries should do well; mine just are not mature enough yet. My apple trees and peach trees produced, too. And my cherry trees--though probably too young to fruit--did make flowers and look healthy. I'm also pretty new to gardening, so the fact that my green beans, squash, zucchini, carrots and beats didn't grow that big may be due to a whole lot of factors aside from my slope.
Supposedly north-facing slopes are good for plants that like to fruit early and then have their blossoms die due to a late frost. Compared to my parent's place that's flat and a few miles away, everything on my property blooms a week or two after they do at my parents, since the sunlight is less and the trees aren't convinced to come out of dormancy as soon.
So, questions to ask yourself are: Where are my sunniest spots? How much sun does a place get in the summer compared to the winter? Do your surrounding trees lose their leaves in the winter or not (mine mostly do not)? If most of your trees are evergreen, I think you can pretty much determine your sunniest summer spots by what gets the most sun in the winter. Also, watch during the winter to see where there is frost and where the frost/snow melts soonest. If an area melts sooner than others, it might be a good candidate for your garden.
If you get full sun from 10am to 2pm you only need 2 hours of additional sun to qualify as full sun. So don't worry too much about sunlight, in fact being on the north side actually makes it possible for you to push your luck and grow early blooming species/cultivars that would have all their blossom killed by frost on the 'south side' in your climate (apricot in zone 6/5 anyone).
Now if you are just looking for a list of shade tolerant plants their is a long list. blackberry, raspberry, goumi, arctic kiwi, pawpaw, honeyberry, cornus mas, currants, gooseberry, elderberry, etc
North facing areas get a lot less winter and early spring light and they tend to be cooler. This means that fruit trees planted there will bud several weeks behind the same tree planted in a sunnier south facing site. If you live in a region prone to getting late frosts you don't want your fruit trees to bud and flower too early. If that tree on north facing site is still dormant when a late frost hits no harm is done. Where as the tree on a south facing site that has already budded or flowered will have the flowers killed off by the frost and thus no fruit that season.
Like Chris mentioned with the fruit trees budding and flowering too early, trees on the south slope can also be outright killed by having their sap run too early in the season, if there is a warm spell in late winter when it is not quite spring yet, and then there is a late winter severe cold snap that expands the sap and cracks the tree. Planted on the north side of a house, the trees are shaded until the season has progressed to true spring, thus the angle of the sun is over the house and the fruit trees can safely flow sap, bud, and flower. Where I live there is still a chance of frost killing flowers at any month, but having shade in the late winter, and even early spring is a good idea, and a North Slope for fruit can be far better than a south face.
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