First in regards to force powers, quote from Jedi Academy:
"Powers from both the light and the dark side will be open to you. Remember it isn't the powers themselves that are inherently good or evil, it's how you use them."
That Kel Dorr Jedi master, I forgot his name, uses something called 'electric judgement' on those he feels deserve it. Its essentially force lightening.
In the EU, Jacen Solo used force lightning in the Vong War, and so did Luke later on, he blasted a Vong who was about to kill Jana (I think) with green lightening, just protecting his loved ones.
Luke also, when he had enough of Vader in ROTJ, seems to have temporarily fell to the dark side when he mowed Vader down, but this was when Vader threatened to turn Leia to the dark side - Luke could not let that happen. Palpatine misinterpreted this flurry of anger as having fallen to the dark side...he was wrong.
I noticed some people mentioning the novel Dark Rendezvous. Its a great novel revelaing much about Yoda. I remember him telling one of the kids that he was NOT a stranger to the dark side, and he was familiar with it.
Remember this is a Jedi that is 900 years old, he must have been through a lot during all those years, including temptation if not outright submission to the dark side (at least temporarily). We all go through it ourselves and we don't even have the power of a Jedi.
To me its simple. The force is not light or dark, but characters have both in them. Power corrupts. I don't know that absolute power corrupts absolutely, but when people get great power they are often tempted to do very shadey if not outright terrible things.
I beleive a true Jedi Master, all of them, would have been tempted by the dark side, would have gone through that trial of power and come out of it without letting their power control them. Even Yoda.
We are the music makers, and WE are the dreamers of the dream. -Willie Wonka