Does anyone know? Thanks..
I am pretty sure that is Austrocylindropuntia subulata cristata. A crested version of the Eve's Needle cactus.
This cactus is a really fast grower which is very moisture-tolerant. If you want to keep it crested you will have to be on the lookout for little non-crested "arms" that will pop up. You want to rub them off when they're as little as possible so you don't scar the plant too much.
Interesting to see those comments. I found it to be a really slow grower here under dry conditions (occasional water). My suspicion is that I should just have watered more. I went so far as to remove the plant in favor of something more exciting. One thing I did notice with respect to the non-crested "arms" is that whenever I pruned the plant to remove them, I often saw more non-crested growth sprout from near the cut. The plant responded to injury by sprouting, basically. Probably less of an issue if you're vigilant, but something to be aware of.
My experience is also a slow grower (as opposed to the non crested version which is a speed demon) and I lost both my specimens due to rot related to presumably too much water... It did not get any more or less water than most of my other cacti, so I hope the advice of lots of water is true, I take a little care with the amount of water, increase it slowly to see how the plant reacts.