xmllint is described as 'command line XML tool' and is an app in the OS & Utilities category. There are three alternatives to xmllint for Windows, Linux and Mac. The best alternative is Xidel, which is both free and Open Source. Other great apps like xmllint are XMLStarlet (Free, Open Source) and XMLViewer (Free).
Xidel is an open-source command line tool to download x/html pages and then extract data from them using css 3 selectors, xpath 2 or pattern-matching templates. Kind of like a combination of wget with xmlstarlet.
XMLStarlet is a set of command line utilities (tools) which can be used to transform, query, validate, and edit XML documents and files using simple set of shell commands in similar way it is done for plain text files using UNIX grep, sed, awk, diff, patch, join, etc commands.