Since it is short enough to fit in the HN title, I did not put it on GitHub or something.
If you are not familiar with bookmarklets, here is how you use them:
1: Bookmark any page
2: Right click the bookmark and chose "edit"
3: Give it a nice name (Like "Alexa")
4: Paste the code into the url field
5: Prefix it with "javascript:"
So the url looks like this: javascript:location.href='https://www.alexa.com/siteinfo/'+location.hostname+'?ver=classic'