Reference Guides:

Lead Implementation Guide
[Could / Long / Salesforce.com]

Campaign Management Implementation Guide
[Could / Long / Salesforce.com]

Products, Price Books, & Schedules
[Could / Short / Salesforce.com]

AppExchange Packages:

Lead Scoring:  This package gives organizations the ability to systematically score leads based on a list of defined criteria.

Account Based Pricebooks:  This package will allow you to assign a price book based on the account, instead of the user manually selecting the price book.

Know Your Enemy: This package is designed to give organizations a more robust method to track competitors.

Lead Convert Chatter: This package will transfer the chatter feed of a lead during conversion.