Here are are several guitar speed drills that I've provided below (in both standard notation and tablature) that can help you improve your speed as a guitarist. Some exercises focus on the picking hand, others on the fretting hand, and still others on coordination between the two.
Practice these exercises slowly at first (ideally with a metronome) and focus on playing them cleanly and smoothly before you increase your speed, otherwise you'll just be learning your mistakes.
Remember the phrase, "Practice makes perfect"? It's dead WRONG! If you practice something wrong, you'll learn it wrong. PERFECT practice makes perfect. So learn these exercises at a tempo that allows you to play them cleanly and only speed up when you've mastered a slower tempo.
Speed Drill #1 is a classic guitar technique exercise that is excellent for developing coordination between your right and left hands and ensures that you are using all fingers on your fretting hand, including the little guy at the end! The example below shows the ascending and descending versions in first position.
An excellent guitar speed drill as well as warm-up exercise is to take this 4 note/4 finger pattern and play it in every position up and down the neck. It might take a few minutes to do so, but it's an excellent exercise and will let you get some practice playing every note in every position.
You can also vary your picking hand to get a lot of mileage out of this one exercise. You can use strict alternate picking or you can pick the first note and use hammer-ons and pull-offs for the rest (or some variation). You can begin with downstrokes or upstrokes to get practice playing in uncomfortable situations. Really, there are numerous permutations. It's written in 8th notes, but 16th notes work just as well. Always use a metronome to ensure you're playing in good time!
One last tip! If you don't know the names of the notes on the fretboard cold, you can use this exercise to help. Play it at a tempo slow enough so that you can say the names of the notes as you play it. Just for the sake of convenience, I've notated it with sharps while ascending and with flats while descending. Say the name of every note as you play it up and down the neck, no matter how slowly you have to play it. This is how I learned the names of the notes on the neck, and it works!
Speed Drill #2 is a variation on speed drill #1. Everything applies here as well. Have fun!
With Speed Drill #3, things start to get a little different. You might have to slow down a bit with this one, but it's good preparation for what's to come!
Speed Drill #4 gives you some practice with triplets and 3 note patterns. This is also a good exercise for economy picking.
Speed Drill #5 is one of the best speed drills around. It uses every possible permutation of a 4 note/4 finger pattern. Note that in 12 bars you're still in first position on the same string! If you've got the time and the course, attempt this exerise on every string and every position. When you alternate between alternate picking and legato techniqe, this exercise can really help you build up some chops!
Finally, Speed Drill #6 is a great exercise for working on your 1 note per string technique. Take this one up and down the neck using clean alternate picking, and you'll be doing some serious work!