January 1, 2010, was Public Domain Day. At the start of each new year, the copyright terms on certain authors’ works expire and those works enter the public domain. There are no longer any restrictions on uses of those works. There are, depending on the nature of some works and the jurisdiction of the works, some exceptions. Notable authors whose works entered the public domain this year include William Butler Yeats and Sigmund Freud. To find more works in the public domain, search www.publicdomainworks.net.
Copyright law today gives an author exclusive rights to her/his work for the duration of her/his life plus an additional 70 years. This means works can be restricted for upwards of 100 years or more. Authors can remove (as well as transfer or sell) any or all restrictions as they see fit. Recognizing that users want to freely and easily access and build upon their works, some authors are using tools such as Creative Commons to remove restrictions. Some are even dedicating their works to the public domain well before the life + 70 year term.
Copyright law and copyright terms have changed numerous times over the years. When you add in unpublished and unregistered works, digital works, and works whose copyright owners are unknown, things can get confusing. Cornell University has a great resource that tells you when certain categories of works have or will enter the public domain.