My favorite Eureka moment occurred a few years ago when I was working on my first novel. In the story, my main character is seeking revenge. However, he is completely, utterly unsuited to violent acts of vengeance; his temperment is gentle. Everyone in the novel sees this but him. Still, he thinks he wants revenge.
I knew that wasn't really what he wanted, but I didn't know what he did want. I puzzled and puzzled 'til my puzzler was sore, with no luck.
At the time I worked in a bookstore. One day while straightened kids' pictures books I came across a (then)new title by Patrick McDonnell, cartoonist and animal activist who created the adorable strip Mutts. He had done a couple books before and I thought they were sweet, so I skimmed through the new one, Just Like Heaven.
In the book, the cat Mooch falls asleep under a tree. When he wakes up he is surrounded by fog, and since he is a cat and not really capable of critical thinking, he decides he must be in Heaven. He then takes a tour of 'Heaven', seeing his familiar places with the idea that they must be Heaven. The idea, of course, is that Heaven is a lot like home.
Reading it, I realized what my character secretly wanted. He wanted Heaven. More specifically, he wanted home.
It was brilliant. I was nicer to customers than I had been in weeks. I couldn't stop smiling. EUREKA!
As much as writing can feel like banging your head against a brick wall, these rare moments of clarity make it all worthwhile. So keep on truckin', waiting for the next Eureka moment.