S.O.
Hands down the answer for me is definitely both of my BOB Revolution strollers! We got a single Revolution when just had DS and now that we have two kiddos we have the Revolution Duallie. Everyday life as a SAHM would be so much more difficult for me if I didn't have these strollers! This thing rolls over EVERYTHING! It makes going to the beach a breeze!
Another one is Kirkland baby wipes. I have them stashed all over the place and use them for everything from baby's butt to wiping down the inside of the car after food spills.
For personal use: I really love my cloth sanitary pads. Easy to use and care for and I am not putting more waste into the environment with disposable ones.
Another personal use one that I can't live without are my EARPLUGS! Now that I am a Mom I can hear EVERYTHING at night. And my DH snores. So these earplugs are saving my sanity and probably my DH's life since I would often slug the you know what out of him in the middle of the night for snoring :D.