I know someone will probably think I'm evil for this, but it's a trick I used in highschool and was the "parent" for my three younger sisters: alone time. Everyone goes in their rooms, those old enough keep the door closed, those too young, have the door open but the baby gate from the stairs keeping them in, and they entertain themselves for 30-45 minutes. Do whatever you want, but I HAVE to do my homework. Put toys down for the baby, and the older kids are big enough to color, write, or play house until you are ready to not lock yourself in the basement.
I also sometimes gave them each a share of a ream of printer paper, a box of crayolas, and for the older one, a few different colored pens from my backpack. I told them to write a suprize book that no one could see until it was finished (in other words, don't call me to help you, you can do it, if you call me I might see your book and then it won't be a suprize --OH NO!!!--)! When time was up, we would staple the pages together and give them to mom/daddy!
I also would (rarely) send them to put swimsuits or just undies on, pop all the knobs off the stove, lock the oven and fridge, call them in the kitchen, give them a towel to sit on, a bowl with about two cups of water and five to ten drops of food coloring - red in a red bowl, blue in a blue bowl, etc, or any color in a black bowl (think very light, but will show on white), a few white crayons - the cheap kind, one thick baby paint brush each, and a thick stack of paper to share. I told them if they colored on the white paper, and then "painted the water on", something special would happen. Baby gate them into the kitchen, and write my essays. For a special effect, I was let them see me pull the bowl out of the cubbard and put the clear tap water into the bowl which I had prepared ahead of time with the food coloring. This I didn't time limit, sometimes they would play with it for over an hour. When they were all done, we used the towels together to dry up the floor. One rule, if you stand up, you don't get to play anymore, it's alone time in your room.
I can't think of any of my other sanity savers. Sorry.