There are no diapers on this list - those should be a given for a diaper bag.  No toys, teethers, snacks, or change of clothes either, though all of those have a home in my bag.  Some of the items below might not be "essentials" necessarily, but they are all items that have worked wonders for me. 

1.  Patemm Changing Pad // If you get a diaper bag that comes with a changing pad, most likely that changing pad is going to leave much to be desired.  I replaced mine with the Patemm changing pad, one that's made in the USA of eco-friendly, PVC-free, CPSIA certified laminated 100% cotton.  Its large round shape covers more of the germ-y public changing table, or whatever surface you have it on, and the design is ingenious: the pad features inside pockets for diapers and wipes and folds into a compact carrier - using one hand - when not in use.

2.  Planet Wise and Itzy Ritzy Wet/Dry Bags //  Wet/Dry bags are one of my most practical baby purchases by far… waterproof, leakproof and odor proof, they are great for transporting dirty diapers or wet clothes.  They are also fantastic for transporting non-wet items and merely keeping things organized in the diaper bag - in fact, most days I use the large size as my diaper bag and just stuff it in my purse.  Both brands are eco-friendly and free of PVC and lead but the Itzy Ritzy bags are BPA free and FDA approved for food storage.  Something to keep in mind if it matters to you - Planet Wise are made in the US and Itzy Ritzy are made in China.

3.  Tutim NYC Toy Sitter // A vital accessory that ensures toys and teethers won't end up on the ground.  Can also be used with a pacifier for those with a loop handle like the Hevea or Natursutten, so no need to purchase an actual paci sitter as well.    

4.  Summer Infant Biodegradable Diaper Sacks //  Sometimes (most of the time) you can't just put the dirty diaper in a wet/dry bag to bring home.  In those situations, these disposable, biodegradable diaper bags are key.  

5.  Neat Solutions Disposable Table Topper // Not a perfect solution, but the best one I've tried for those times when the situation really warrants a placemat.  We still use a plate on top, but inevitably quite a bit of food touches the table before going into M's mouth.  They seem to stick well, though not to every table surface, and either she can't pull them off or she doesn't want to.  They're also BPA/PVC free and biodegradable.  I do wish they came in a solid color, however, because the cartoon characters distract a bit from the food.     

6.  Satsuma Designs Baby and Toddler Poncho //  A laminated cotton poncho to stash in the diaper bag or car for monsoon emergencies - it can also be used to protect the stroller or car seat from a downpour.     

7.  Mama and Baby! Indestructibles Book // A 100% non-toxic, rip-proof, tear-proof, chew-proof, washable book that takes up zero room in a diaper bag and can be pulled out to entertain baby in a pinch.  

8.  Dapple Pacifier Wipes //  Natural, preservative free, food-grade wipes to quickly and safely clean everything from a dropped pacifier to baby's fingers.  Individual wrapped packets are perfect for tiny diaper bag pockets.      

9.  Under the Nile Organic Sherpa Bibs //  The perfect drool-bib for the diaper bag, these organic cotton bibs are super soft, thick, and absorbent and can double as a wash cloth for small clean-ups.  Now that M is no longer in need of a drool bib, I make sure I always have my favorite Aden and Anais Burpy Bib or a silicone bib for eating (reviews in my Highchair Essentials post).

10.  Honest Company Wipes, Healing Balm and Travel Hand Sanitizer //  I don't go anywhere without these wipes… I stash them in every room of the house, the car, the diaper bag, my purse.  Honest Company does make a travel size package, but it only contains 10 wipes and they annoyingly all come out at the same time.  I prefer to wait until I have used half a full-size package at home and then add that one to my diaper bag.  This diaper cream is my favorite for the diaper bag as it does not leak nor is it too messy.  And the hand sanitizer is a given, but this one smells and feels better than most.   

11.  Buti-pod Wipes Case //  If carrying an excessive amount of wipes isn't your cup of tea, try this refillable case that holds about 15 wipes and keeps them moist.      

12.  Itzy Ritzy Highchair and Shopping Cart Cover //  Maybe not so much something you want to store in the diaper bag, but definitely something that could be kept in the back of the car or stroller.  A machine washable, CPSIA certified padded cotton cover to add comfort and protect from germs on highchairs and shopping carts.  Also includes straps to attach toys and teethers.