There is no free version of Salesforce, atleast in commercial sense. With one exception though; If you are a non-profit organization you can sign up for the nonprofit edition of Salesforce which offers the first 10 licenses free - for qualified non-profits. Salesforce.com is resourceful.
Also question is, can you pay monthly for Salesforce?
Salesforce will pressure you to pre-pay for two years, but they'll settle for one year, never monthly. The monthly subscription rates listed by Salesforce at http://www.salesforce.com/crm/editions-pricing.jsp are predicated on annual payment, since there is no monthly payment option.
What are the licenses in Salesforce?
Designed for users who require full access to standard CRM and Salesforce AppExchange apps. Users with this user license are entitled to access any standard or custom app. Each license provides additional storage for Enterprise, Unlimited, and Performance Edition users.
What is Salesforce Enterprise Edition?
Includes all Enterprise Edition functionality, Premier Support, full mobile access, unlimited custom apps, increased storage limits, and other features. It lets developers extend Salesforce, integrate with other applications, and develop new tools and applications.