If you have worked on Salesforce before Force.com Free, you know the value of a good old Developer account for hand-ons Salesforce but with Force.com Free Edition in picture many new users are confused what should be done.
So if you are new to Salesforce and wish to play around Salesforce, don't get confused. Here is a quick guide what to do...
Free Force.com is a Force.com license, that means you can build/Deploy and create custom app like VolunteerForce but no Salesforce standard CRM Clouds (Like Sales Cloud, Service Cloud)
You may not feel the difference with one use as you as System Admin and can see these clouds, but any other user you create in the org won't have access to them.
Developer Edition, the beautiful and my favorite one (because it is relatively old and happening) is a test org, essentially its like your personal sandbox. A developers best friend, when you want to test code, Apex, Visualforce and stuff.
The trial version is just for you to study Salesforce wheather you want to buy it or no. This is a pure business decision. If you want to try Salesforce.com for your company, go for trial version and then convert to EE or UE after your business decision. If you are a developer and want to play around SFDC go for Developer Edition.
And if you are not concerned about CRM at all and just want force.com platform, Force.com Free Edition is there for you.
This post is going to help you only in chosing between free editions, for making a decision of paid versions, you might want to check the Salesforce Edition Pricing
Between Enterprise Edition and Force.com Enterprise edition
Salesforce.com is a CRM with all the standard CRM features, like leads, accounts and opportunity. While force.com is a platform for you to build your own database driven applications like HRM, Recruiting, Banking or even Volunteering Application.
The pricing of Force.com is much cheaper than Salesforce.com, so if you don't want CRM, you can save big bucks there.
Go into details of pricing at the Salesforce Edition Pricing Comparison page.
Oh and Btw, Happy New Year.
Hope it helps,