I use the Unarchiver as well, just so i can tell it were to unzip the file if I don't want it in the same folder as the original. I also have BetterZip for the same reason.
I thought I remembered that the zip support was built in because I was unzipping things before I found the other programs.
I wonder what you mean by "open." But if I want to uncompress a zip file's contents, then in a Terminal window or in an xterm window, I type something like this:
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