Most of the online free resources I found are for beginners and does not provide much information. Is there any more comprehensive resource covering from beginner to advanced level?
You are right, the dominant type of material out there on the web is beginner and/or very poor quality. I have used GermanPod101 extensively.They have lots of intermediate material and some advanced dialogues. You can also use SlowGerman.com podcasts for listening comprehension practice. I've used Yabla.com at times and, of course, free language exchange via Skype or other is very good. Look at MyLanguageExchange, iTalki, Busuu, etc.
You can't go past About.com's About.German. They have lots of lessons and lots of good special sections on grammar. There's a fantastic podcast series called GermanGrammarPod the literally dissects German grammar point for point. It's excellent, but very dry... you have to really be ready for it. But it is something I go back to time and again. Really good stuff!
Some of what I've listed above is not 'free'... but none of it is expensive. For example, you can get all of the GermanPod101 material to date for a one month premium subscription. If you download all the podcasts to your iTunes (or whatever), you can hen not renew and it's yours! If you are new to them, it will take you months to get through their material!
I have recently started a website I'mLearningGerman.com to try and help people looking for good material. It's early days yet, so not a lot there. But please consider checking in with it, as over time it will build up and hopefully become and honest home for quality help for people like you looking to learn German online. That's my commitment!
Looked at http://www.duolingo.com? I like it for French, and have done a bit of the German and Spanish as well. Gamification, reminders, following people. Addictive learning.
(No affiliation. Just a happy user.)