More commonly you'll find shaving gear (be warned, though, the razors are usually so crap they should come with bandaids) and toothbrush sets that I always take because they are just the ticket when guests forget their own.
And I'll probably never stop dropping a few individually wrapped mints into my handbag when confronted with a bowl of them; I'm sure I'll want fresh breath when the day of reckoning arrives. As for single-serve green tea bags, I take them too; you can never have enough green tea.
And don't get me started on the deep affection I feel for a breakfast buffet when it's included in the price of the room, which makes it feel like it's free even though it's not, really.
Travelling alone, I'll think nothing of pocketing a few boiled eggs and some fruit for later. And the money saved can be squandered later on souvenirs.
As for travelling with a child - I have been known to take Tupperware down to the dining room and surreptitiously make an entire day's picnic. The prospect of a hungry child can make a parent utterly shameless. Croissants smeared in Nutella are surprisingly delicious.
For all my fondness for free stuff, though, I need to make it clear I have never taken a robe, although I love how they're so fluffy and clean. I have taken several pairs of those silly slippers because I don't think they can be re-used, yet I never use them when I get home.
I once borrowed an airline blanket (having not given any thought to sleeping equipment before three months' backpacking around Central America) and goodness knows what super stuff it was made from but it was no worse for wear after months of being used as my towel, sheet and modesty wrap. I left it on the plane on my way home.
I suspect the reason humans are particularly partial to pocketing things while travelling is because, having just forked out for the flight and the hotel and all the other incidentals, by grabbing a few shampoos it feels like you're recouping some of those expenses. Which is rather ridiculous when you think about it, so from now on, the only thing I'm going to take while I'm on holiday is photographs.