I'll be honest - my handbag is a bit Mary Poppins-ish. I throw so much stuff in there and constantly have to root around to find what I want. But, there's always a few products constantly sitting in there that only leave when they run out.
First up is the Caudalie Beauty Elixir. Admittedly I don't feel like this does anything special, but a few spritzes really freshens the face, particularly in summer when it's hot and clammy. The 30ml is a also a nice handbag friendly size, so it doesn't take up too much room.
Next is a Bath & Body Works hand sanitiser. I use these things all the time - I have one in my handbag, one on my desk at work and copious amounts sitting at home from when I stocked up in Florida. My favourites are the Christmas scents though, which you can get on eBay for around £2-3. They're much cheaper in the US (5 for $5) but the scents are what makes them stand out to the hand sanitisers you can get over here.
Finally, a good lip balm is always a necessity. My lips suffer in Winter, so I have to make sure I keep a particularly nourishing lip balm in my bag to keep them hydrated. I love the Nuxe Reve de Miel in the pot, but it's not as travel appropriate as the tube, so I tend to stick to that one. A summer favourite is the EOS lip balms, but I find they don't condition the lips as much in Winter.
It's also worth mentioning that I normally keep a hand cream in my bag too, but they're currently sitting on my desk at work as that's where I tend to use them most. I love the Laura Mercier hand creams, they smell amazing!
I'd love to know what your handbag essentials are!