Current Salesforce.com documentation. If you're interested in older versions feel free to contact me.

Release Notes - Spring '12

Salesforce Limits Quick Reference Guide

Force.com Sites Implementation Guide

Force.com Platform Fundamentals

Formulas Quick Reference Guide

Using the Drag-and-Drop Report Builder

Tips For Creating Dashboards

Useful Formula Fields (HTML) (PDF)

Useful Validation Rules (HTML) (PDF)

Useful Workflow Rules (HTML) (PDF)

Workbooks

Analytics Workbook (new!)

Force.com Workbook

Visual Workflow Workbook (new!)

Visualforce Workbook

Apex / Visualforce

Apex Code Developer's Guide (HTML) (PDF)

Visualforce Developer's Guide (HTML) (PDF)

Developer Cheat Sheets

Visualforce Cheat Sheet

Formulas Cheat Sheet

Chatter Cheat Sheet

Apex Code Cheat Sheet