Where to find type of edition in Salesforce org?

Click on Your Name -> Setup -> Administration Setup.
Top of the loaded page you can see which edition you are using. .

UPDATE
Removed the WSDL checking option to identifying the Salesforce edition based on @KeithC's comment. Checking at the Administration Setup is the best option to identify the edition.

Once you have successfully logged onto SFDC ORG, the browser window/tab title (Salesforce.com - will provide you with the edition like Developer, Enterprise, Unlimited, etc. you have logged in.

For Firefox (refer below)

For IE (refer below)