The Personal Code: A character personality questionnaire by Alesia Chamness.
The Mother Of All Character Questionnaires: A huge list of questions that will help you create a character, from Role-playing Tips.
Ideas for Character Backgrounds: Another useful character questionnaire, from hurva.org.
Guide to Character Personality and Background, by Ash. A clear and simple step-by-step guide to developing a character's personality. This process leaves you with a sort of role-playing character sheet that you can bring to a game as a reference.
The 100 Most Important Things To Know About Your Character, by Beth Kinderman and Nikki Walker.
The Wizard's Apprentice: The life of an apprentice. Useful for a wizard's character background.
Palladin Murderers: Creating a less stereotypical palladin.
5e Character Builds from a variety of sources.
Getting in Character: I was having trouble playing a new character, so I asked for advice.
Errant Dreams: This site has articles on character creation and world building.
Roleplaying Tips: A free online weekly magazine about roleplaying.
Character Sheet: 3E Wizard: A super-dense, three-page D&D 3E character sheet in MS-Word format especially for wizards, by me. Includes standard 0th level spells and a polymorph section. The most confusing parts have been filled in with example values.
Character Sheet: 3E Finesse Fighter: A super-dense, one-page D&D 3E character sheet in MS-Word format especially for finesse fighters, by me. This can be used for any non-casting class, but the armor section is reduced for finesse fighters. The most confusing parts have been filled in with example values.
Character Sheet: 2E: A super-dense, super-ugly AD&D 2E character sheet in RTF format, by me.
Official D&D 3.5 character sheet, distributed by Wizards of the Coast.