Evepraisal is a tool commonly used to check prices of goods to sell in eve at various trade hubs. Additionally, its permalink function has become a standard when forming contracts in game, allowing the other party to see what you see and verify the price that you were quoted via the site.
EveMarketer is another tool to check prices, but is not limited to just the major trade hubs. You can look up any item and see where it is being sold, and for what price.
Eve Gatecamp Checker allows you to input a beginning and ending system (as well as any waypoints you may need to travel to) and instantly see what the status of gatecamps are along your route. You should ALWAYS assume gates near tradehubs are camped, but this tool can give you advanced warning of systems you may want to avoid, or even delay your trip to a safer server time.
Dotlan is a map service giving you vital information about the systems of eve including infrastructure, sovereignty, and security level
Eve University is a wealth of knowledge about the game. Don't know how to do something, look it up here!
EVE PI Relations is a useful tool for those who are in Planetary Interactions, highlighting the connections of different PI levels