I live in zone 6b in Amesbury, a small town/city bordering NH and one town over from the Ocean. We have a bungalow house, with a deck to a a smallish yard, on the south west side. I have planted three pawpaws, growing nicely, and have an apple espalier, sunchokes, currents, strawberries and raspberries. I'd like to plant more yield bearing trees, should they be fruit or nut trees, or do you have other suggestions?

Welcome to permies Deb!

The Plants For a Future Database is a good place, I think, for coming up with ideas; selecting USA zones 2 or 3 in the search results and then adding whatever other filters you're looking for can narrow down your search.

For limited spaces, I think that hazelnuts are a good choice:
They are usually not as large as most other fruit/nut trees.
They begin producing quicker than most fruit/nut trees.
They seem to do well in many locations - many fruit/nut trees have special climatic conditions, yet the hazelnut seems to fit just about anywhere.

Hi Deb,
you can squeeze a lot in if you try! I'm only talking about space-bear in mind my climate is temperate.
In my small place-sort of possibly about 2000 square foot-
I have two espalier semi dwarf apples, an espalier double-grafted pear on quince dwarfing rootstock, a
vaguely fan-trained multi-grafted plum, a seedling peach that I'll brutally prune, ditto another apple,
A mountain pawpaw, two types of raspberries, thornless blackberry, boysenberry, two feijoa cultivars, black passionfruit, fig...

Anything that I can grow flat, and/or up, I do. You're already growing espalier, so you're clearly well on your way

Check out these guys for ideas they have a huge selection of plants that only get to 8 to 10ft at maturity with no pruning.
https://www.onegreenworld.com/Aronia/338/

Aronia @6ft
Juneberry @6ft
Patio dwarf peach @6ft
Patio dwarf nectarine @6ft
Grape on a 6ftx6ftx6ft arbor
Medlar @8ft

It is good to see a fellow Mass on the forums.

Here is a group of permies that you can meet in person and trade ideas.
http://www.meetup.com/Boston-Permaculture/

I am pretty sure there is another group for the north shore too.

I like plum trees because they are so resilient. They can be pruned to suit your needs. Suckers make great garden stakes. Squash, beans and others can climb up the plums, which keeps them safe from bottom rot and saves space. Many with smaller spaces, forgo important crops, due to their sprawling nature.

Any fruit tree you like! All of them on really small space.
The right rootstock and/or intensive pruning keep trees your hight.

Prunus tomentosa (Nanking cherry) it is one! :)

These guys have some interesting ideas on maximizing fruit trees in a small space...which you probably already know, from your espaliers.. but..

http://www.davewilson.com/home-gardens/backyard-orchard-culture

I really like American persimmons. They can grow tall and spindly and still produce loads of fruit without having to stretch way out. They're also so beautiful, delicious, and easily preserved. We have self pollinating varieties here on my farm and nursery. They are one of my top 3 favorite trees to plant (chestnut and mulberry are the other two).