Workbench - Login selecting your environment and salesforce credentials, then use any of the numerous tools including SOQL Query editor, REST explorer, and more.
Functions - Docs for using functions in either VF pages or formulas. Very useful page to have bookmarked.
Cheat Sheets - Cheat sheets for various aspects of Salesforce including Apex, Visualforce, Formulas, and Chatter API
URLFOR Finally Explained - Excellent blog post on URLFOR and $Action used in VisualForce
dataloader.io - Browser based tool for loading data into SFDC