Links I refer to all the time.

Editing marks

[not quite clear who this belongs to]: Standard proofreading marks

Special characters

Arnold Winkelried’s special characters tip sheet: One of the few that includes both HTML name/number codes and Mac/Windows keyboard shortcuts

Left Logic’s HTML Entity Character Lookup: Elegant lookup based on visual similarity – type in o and get codes for °, Ø, ö and σ

Type

Identifont: Identify a typeface by answering a series of questions

WhatTheFont: Identify a typeface from an image

Colour

Eric Meyer’s Color Blender: Sometimes you find exactly what you’re looking for on the web – an RGB colour blend/tint calculator which accepts 0–255, % or hex values and gives you up to 10 intervals between the colours you specify.