Here is the release overview presentation with screenshots highlighting the important features and functionalities introduced in Salesforce Winter 16

Here is the release overview presentation with screenshots highlighting the important features and functionalities introduced in Salesforce Summer 15

Here is the release overview presentation with screenshots highlighting the important features and functionalities introduced in Salesforce Spring 15

Here is a list of all new features and functionalities (with screenshots) introduced in Salesforce Winter 15 release.

The following presentation shows important features and functionalities introduced in Salesforce Summer 14 Release with screenshots

The following presentation shows important features and functionalities introduced in Salesforce Spring 14 Release with screenshots

Salesforce.com Spring 11 Release Preview Videos