I found these in a couple of fruit markets in Hong Kong. When I tried to ask one of the vendors what they are called, she said something that sounded a lot like lemon (lih mah), but I don't think she had a very good grasp of English.
From rooting around on the internet, I gather these belong to the genus Syzygium. Can anyone identify them more precisely?
The riper one of the two that I had was sweet, very crisp, and fully of water. I imagine these would make for a very refreshing treat on a hot day.