Context for my recommendation: I am currently a practitioner of UML and have been researching the usability of UML for my Masters.
Oddly enough most online tutorials are not well written enough and focus on teaching you the syntax rather than the semantics. You really do not pick up UML in 1000 words and 10 screen shots. Would strong advise to give in and get a book like Martin Fowler's "UML Distilled: A Brief Guide to the Standard Object Modeling Language". This book is used by many colleges and universities and is supported by the standard owner OMG. UML can be overwhelming and you need to slice it up into the important parts and then build on that, which Fowler's book does very well. Fowler focuses on what you need, diagramming, but does not short change the modeling or theory.
If you are up to speed Fowler's cheat sheets which are at the front and the back are worth the price of the book, but there are some online close enough versions."http://dn.codegear.com/article/31863"
What Fowler will not provide you with is strong theory, modeling, MDA information, but often his book is more than sufficient.
Do not go to the OMG specs unless you have an extremely detailed question as you may not even be able to find it.
If you are really committed to a free online tutorial read about five of them and print out a good summary that you can use as a cheat sheet.
I would stay clear of the OOAD book recommended above as it teaches OOAD which is a process/methodology and not UML (A language).