Every January 1st, the copyright expires on a year's worth of published literature, giving us Public Domain Day. Exactly what this means is different in different countries, as you can see on Wikipedia's list or Public Domain Day's map. In Australia and much of Europe and South America it essentially means that works from authors who died more than 70 years ago are now in the public domain; in New Zealand, Canada, and much of Asia and Africa works from authors who died more than 50 years ago are now in the public domain.
(The USA is more complicated, which unfortunately constrains US-based websites like Project Gutenberg and Librivox.)
A great place to browse what's available in the public domain is at the Public Domain Review, created on Public Domain Day 2011 - a smorgasbord of "exotic scraps and marvellous rarities" to bring attention to well-deserving items that might otherwise languish in obscurity.
