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

Salesforce Youtube Channel - Contains videos related to Salesforce and its products. In particular, videos to Dreamforce sessions including Apex Design Patterns and Best Practices

Data Loader- Everything you could ever want to know about the Data Loader including using the DL from the command line.

dataloader.io - Browser based tool for loading data into SFDC