One of the most versatile and economical sources of protein you can serve is beans, especially when you cook them from scratch using dry beans. A great place to buy them is at stores like Whole Foods that have bulk bins, and organic beans are only slightly more expensive. Bulk organic beans at the Whole Foods close to me are actually significantly cheaper than the pre-bagged regular beans at the grocery store.
There are many different types of beans (and lentils and dried peas), all of which have distinctly different tastes. The ones I cook the most are black beans, pinto beans, Great Northerns (medium sized white beans), and small white beans. Many of them are interchangeable in recipes. I usually cook them in the Crockpot, but they can easily be cooked on top of the stove, and they freeze beautifully. I freeze them in freezer-safe wide mouth canning jars, and the one pint size jars are roughly equivalent to one can. I make a large batch and freeze the extras.
The basic cooking method is the same for all types of beans and requires nothing more than beans, water and salt. However, there are many variations that make them much more tasty. Start by picking over the beans to remove any rocks or other debris. Rinse well and cover with plenty of water (they will more than double in size) and let sit overnight. (This step is not essential, but makes the beans cook faster and be more digestible.) The next morning, drain all the water off, put in Crockpot (I soak mine in the Crockpot liner) and add fresh water to cover, Cook on high until tender, then add salt to taste. Different sized beans, lentils, etc. take different lengths of time to cook, but plan on 4 - 8 hours. When cooking on top of the stove, bring to a boil and then simmer until tender (a shorter cooking time). If the beans are old, they may take a longer time to cook.
Just a few ideas: use broth (any kind) for part or all of the liquid, add meat (bacon ends & pieces, smoked ham hock, bone left over from picnic ham, turkey sausage, etc.) to beans at start of cooking period, add diced onion and/or garlic at start of cooking - let your imagination be your guide. Do not add salt until after beans are tender - my Dad always said that it kept the beans from getting soft if added too soon. A bowl of beans, especially cooked with meat, and a side of cornbread makes an outstanding and economical meal.
Refried beans made from scratch are very versatile and also freeze well. I usually modify the linked recipe slightly, leaving the onion in the beans and pureeing in a food processor. The amount of water for the cooking stage will be different, also, since the beans soak up a good bit while they are soaking.
A favorite way to use refried beans for a quick and tasty meal is to spread a generous spoonful or two of beans down the center of a flour tortilla, sprinkle with some shredded cheddar cheese, fold both sides over, and fry until golden brown in a little oil, starting with folded side down and turning halfway through cooking. Don't use too high a heat - the inside needs to get heated through by the time the tortilla browns. (I like to add a little leftover meat or a cooked piece of bacon to the beans.) Eat as is or add chopped tomato, shredded lettuce, crumbled feta cheese or sour cream. I learned to fix this from Mexican friends when we were living in Mexico (they usually just used beans and tortillas), My kids loved these "fried burritos" when they were growing up and it was also something the kids could fix for themselves when they were old enough to safely use the stove.
What are some of your favorite ways to use beans?