Update September 2016: it appears this device is been discontinued. You may still find it at some resellers or move into a house that has one so I will leave this post here, but I don’t think you’ll be able to find one new.
(US only?) I just realized we’ve never covered the Enerwave SC7 in this topic because originally I had thought we were only talking about battery-operated devices. But there are a couple of other devices mentioned in the thread that have to be wired into mains power. So depending on how you define “button” the SC7, which is quite popular in the community, might work for a particular use case.
This is sort of an unusual device in that it has to be wired into regular power, and usually replaces a regular light switch, but does not itself control the load on that circuit. But of course that also makes it more like a “remote” than a typical wall switch. 
It fits well in the space of a single gang box and is sold by most zwave retailers for around $45.
One community member said that Enerwave was displaying an SC8 at their booth at CES 2017, and that it looks basically the same except there are two buttons at the bottom instead of one big one. I’m assuming it will also be zwave plus. But it hasn’t gone through certification yet.