Check out Dave Weiners Youtube channel he covers all the modes there and a whole lot more, the Mixolydian lesson is particularly good as he applies it to multiple styles and explains why it works.
Dave Weiner Youtube channel
This guy tours with Vai and still finds time to make these lessons, awesome.
music.stackexchange.com/users/149/Bella
From personal experience: Piano theory lessons .
I started piano when I was 10 or so, and
guitar when I was 17.
I was amazed at the similarities.
One of the things most people don't really think about is that they're actually both string instruments, of the "struck" variety (non-bowed, though of course you can "pluck" guitar strings).
The most valuable part to me was interval training .
I was basically tone deaf before I did interval training, and now I can play a lot of songs by ear.
This is really valuable with guitar since it's often when you can't find a good tab for a song.
A large portion of music theory is applicable across all instruments -- notes, keys, scales, etc.
-- and an even larger portion is applicable across guitar and piano (all the above plus chords, for example).
I wouldn't worry too much about getting into guitar-specific theory until you've covered all the basics.
music.stackexchange.com/users/None/
In my opinion this is one of the best sites I have found.
Simple, easy to understand, has examples and resources for practicing and memorizing theory, etc.
I feel it also moves very smoothly from basics to more complicated subjects.
I use it as my primary resource.
Guitar Theory
music.stackexchange.com/users/82/Mike Webb
A good majority of the teachers that come into our store use Alfreds Esssentials of Music Theory.
It does lend itself more towards the piano but its a wonderful series and learning that will only add to your knowledge and ability as a musician.
It aslo gives the option of getting just the book, CD, DVD, or some combination of them all.
http://www.alfred.com/emt
music.stackexchange.com/users/None/
Another site worth looking at is http://www.musictheory.net/
They cover a lot of the basics and have some neat tools for re-enforcing the lessons.
music.stackexchange.com/users/268/AnonJr
The best resource may very well be a well educated teacher.
One that can play you melody and harmony exercises for you so that you can hear what works and what does not would be especially useful.
Try getting one with a Licentiate and/or Diploma in theory.
Also post grad work in theory from a good music school is a great qualification.
music.stackexchange.com/users/7306/Neil Meyer
My teacher recommended Edly's Music Theory for Practical People .
I've just started it.
I find very readable and appropriate for a complete beginner.
It has worksheets to complete in addition to explanations.
music.stackexchange.com/users/10879/Noah
I would also recommend Chris Juergensen's lessons .
He has a very informal way of explaining things, and with examples which I found very useful.
There are plenty of lessons on his website and more in his books .
music.stackexchange.com/users/10982/user1953384
... learning music theory, especially as it relates to the guitar.
Coursera has free courses from music colleges / conservatories.
They're growing too
so I would expect new material over time.
Introduction to Guitar from Berklee College is highly rated by it's students and covers some foundational aspects such as major and pentatonic scales.
Fundamentals of Music Theory from University of Edinburgh covers a lot of theory and is very information dense but is also highly rated.
music.stackexchange.com/users/28626/timbo
What is a good resource for learning music theory, especially as it relates to the guitar, coming from a completely beginner's perspective?  I had a musician friend try to teach me about modes such as Mixolydian, but I feel I should start far more basic, and I don't know what is most relevant to modern guitar music.
What are good resources for learning music theory?
music.stackexchange.com
<theory><learning>
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music.stackexchange.com/users/None/
For learning music theory, teachers recommend Alfreds Essentials of Music Theory and Edly's Music Theory for Practical People. Fundamentals of Music Theory  from University of Edinburgh covers a lot of theory and is very information dense but is also highly rated. Introduction to Guitar  from Berklee College is highly rated by it's students and covers some foundational aspects such as major and pentatonic scales.  Online resources include free courses on Coursera,  http://www.musictheory.net/   and Dave Weiner's YouTube channel. Chris Juergensen's lessons are also recommended.  Recommendations include: Alfred's Essentials of Music Theory; Edly's Music Theory for Practical People; http://www.musictheory.net/ ; Dave Weiner's YouTube channel and Chris Juergensen's lessons.  Coursera has free courses from music colleges / conservatories. Introduction to Guitar  from Berklee College is highly rated by it's students and covers some foundational aspects such as major and pentatonic scales.  Fundamentals of Music Theory  from University of Edinburgh covers a lot of theory and is very information dense but is also highly rated. However, the best resource may very well be a well educated teacher.
The best resource may very well be a well educated teacher. Recommendations include: Alfred's Essentials of Music Theory; Edly's Music Theory for Practical People; http://www.musictheory.net/ ; Dave Weiner's YouTube channel and Chris Juergensen's lessons.  Coursera has free courses from music colleges / conservatories. Introduction to Guitar  from Berklee College is highly rated by it's students and covers some foundational aspects such as major and pentatonic scales.  Fundamentals of Music Theory  from University of Edinburgh covers a lot of theory and is very information dense but is also highly rated.