Sling - used so Dh and I could eat dinner when they didn't want to be put down. Great for shopping without crashing pram into everything. Used almost 4 years (great when your 4 year old falls asleep on midnight flight and you have a 20 min walk at Gatwick to security)
Cloth nappies and wipes - sound a faff, actually very easy. Saved clothing as never leaked, saved ££ as £400 lasted 2 children worth. Saved buying each week and worrying your running out.
Baby bug in a rug - a blanket with feet basically. Means they can wear in car seat safely as you strap in then fold blanket over, can wear in sling, in pram, etc. If you go inside and it's hot can open up quickly and they sleep as not having to take full coat off
Random one:
Nairns oat biscuits - come in various flavours, in little packets of 3 biscuits. Can leave in draw next to bed incase you become starving feeding baby at 2am, or good to throw in handbag for same reason when out.