Wikipedia (or rather, the community on Wikipedia) keeps a pretty good up-to-date list here.
Click on this link to see which version your BROWSER is using: http://jsfiddle.net/Ac6CT/
You should be able filter by using script tags to each JS version.
My Chrome reports
In chrome you can find easily not only your JS version but also a flash version. All you need is to type
chrome://version/ in a command line and you will get something like this:
For IE :
Refer Squeegy's answer for non-IE versions :)
All of todays browsers use at least version
Concerning your tutorial site, the information there seems to be extremely outdated, I beg you to head over to MDC and read their Guide:
You may still want to watch out for features which require version
1.6 or above, as this might give Internet Explorer some troubles.
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