The one thing I notice writers having trouble with is naming people, places, and things. In one of the current manuscripts I'm editing, the author has a bastard spelling of a popular Harry Potter name and has had a character give an animal a really awful name by adding -arro to the word describing a particular characteristic of the animal (trust me, it's really bad). Names are important. They need to reflect the character, the character's culture, and not make readers groan in disgust or put the book down laughing. They also cannot be similar in spelling or sound to characters from popular books. It won't matter if you've been working on the book for 20 years--name a character similar to an unusual name in a book that is well known (and made into movies!) and you will be accused of ripping the name off.
I won't get into the cultural aspects here--I get grief over my names all the time even though I make sure there is something shorter and usable for the reader. I will however give some places to go for names. ;)
First, look at the profile pages of some of the really popular LJ's. Don't use the user names as is, but they make a great foundation when playing around, especially for those SFF stories. ;)
Also, if you have a writer's notebook, keep lists of names that you come across and like for future use. Build this up and you'll never run out of names. (And if you don't have a writer's notebook, shame on you!)
Beyond that, try some of these:
Babynames.comBaby Names Around The World20000-names.comThe Random Name Generator <~~ This one is FUN
Medieval Names Archive Kate Monk's Onomastikon (Dictionary of Names) Feel free to add more in the comments for me to put on the Alden.nu resource pages once I get them back online.
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