s0metim3s has an extraordinarily useful collection of links, mostly theoretical, on rights. See also her essay "The Barbed End of Human Rights".
All the major human rights organizations have websites: such as Amnesty International (who have their own collection of links) and Human Rights Watch (see their page on the Americas). See also for instance the European Human Rights Centre.
Some Declarations of Rights...
- Magna Carta.
- US Declaration of Independence, Constitution, and Bill of Rights, including amendments 11 to 27. See also the Library of Congress's site of documents relating to The American Revolution and the New Nation.
- The French Declaration of the Rights of Man and the Citizen.
- The American Convention on Human Rights (note also the signatories and dates of signature and ratification).
On Latin America in general, a good starting point is the Latin American Network Information Center; they also have a page on Human Rights in Latin America.
The OAS has an Inter-American Commission on Human Rights.
On the sidebar I have links to two blogs that, together, do a pretty good job of covering news on the region as it's reported week by week: Latin America News Review and The Latin Americanist.
I also have a few country-specific blogs there: Blog from Bolivia, Tim's El Salvador blog, and Look for me in the Whirlwind (this last, from Venezuela). But there are lots of other blogs from or about particular countries. You can use Technorati to look for, say, blogs about Argentina or Mexico.
(Technorati also lists many blogs that claim to be concerned with human rights; it'd be nice if you could search for blogs that are tagged both human rights and Latin America, but that doesn't seem to be possible.)
From Georgetown University, an excellent project on Comparative Constitutional Studies, with the text of all the various Constitutions of the Americas.
Many of the Latin American truth commissions have published their work online. Argentina's Nunca más is a particularly useful site, with lots of testimonios. (The testimonios are in Spanish, but the published report is also available in English.) Check out too Guatemala's Report of the Commission for Historical Clarification, whose conclusions and recommendations are available. Then for instance there's Peru's Comisión de la Verdad y Reconciliación.