I always have trouble finding the documentations for the following, by documentation I mean good complete ones. Say what you want about the PHP, but it's documentation is the best I've seen so far.
So here's the list. Please let me know of the documentations that exists for these, if any.
Update: Few more added, others might find it helpful.
Their documentation is good, but the tutorials are really basic..
Perl comes with great documentation out of the box especially if you're on Linux using the perldoc command will give you information about core functionality and CPAN modules.
But since you asked for online you can also view the same docs at http://perldoc.org and for modules at CPAN
BTW I have to disagree that PHP's documentation is good or complete. There are too many "gotchas" that you don't find unless you read all the comments!
For the Ruby api, I like rubybrain.com (and it's sister site, railsbrain.com for the rails api). It's just the api, but it's got a nice, humane, AJAXified UI with good search functions.
For general instruction aimed at a newcomer to the Ruby language, The Pickaxe Book by Dave Thomas (of Pragmatic Programmer fame) is available online for free. If you like a little more weird in your tutorial, why's (poignant) guide to ruby is pretty classic.
As a collection of assorted Ruby documentation, Ruby-Doc.org is quite good.