Creative Commons (CC) licenses are an increasingly popular way to let readers know what they are allowed to do with web content such as journal articles without asking for permission. These licenses encourage broader distribution of published research, often enabling it to reach a wider audience.
The Directory of Open Access Journals (DOAJ) now recommends CC licenses as a best practice. DOAJ is regarded as both an important mode of discovery and marker of legitimacy within the world of open access publishing.
The Open Access Scholarly Publishers Association (OASPA) also recommends CC licenses, and CC BY in particular. Their site hosts a licensing FAQ with more information about the reasoning behind their recommendations.