1. Hermione Granger (Harry Potter by J.K Rowling) was always a costume option for me. Growing up I had the long, frizzy golden brown hair of Emma Watson and was often the nerd with her hand up in class.
Check out the-leaky-cauldron.org for a DIY Hermione costume.
2. Alice (Alice In Wonderland by C.S Lewis) is another favourite. I have a beautiful black and white pinafore dress that puddles around me like Alice's does in the Disney movie.
Alice-in-wonderland.net for Alice costume ideas.
3. Little Red Riding Hood is another classic that can look suave, sexy or smart. A cute little dress, a red hood and a basket is almost all you need.
The halloweenmart blog has LRRH ideas.
4. Sherlock Holmes (The Adventures of Sherlock Holmes by Arthur Conan Doyle) isn't too difficult either. Throw on a trench coat, grab a bubble pipe (ala Bart Simpson) and throw on a matching cap.
Howtomakestuff.com has a few suggestions on how to throw things together.
5. Katniss Everdeen (The Hunger Games by Suzanne Collins) is my curent choice. I bought a Mockingjay pin and I'll throw on black skinny jeans, with a black singlet and a leather jacket. I'll buy a children's bow & arrow set, and some fake leather material. I'll use the leather to sew brown boot covers, and wrap it around a cardboard packing cylinder for a quiver.
I'm only doing my top 5 cause I'm super lazy :)