This Sunday is "Day for Darfur" with a New York rally to demonstrate the need to send United Nations peacekeepers to Darfur, Sudan. Visit the Global Day for Darfur page and check out the rallies scheduled for other cities and countries. You can also dress yourself up in a blue peacekeeper beret or sign Amnesty's "add a blue helmet" petition here. If you want to do something a bit more serious, Amnesty International has issued the fourth in its series of actions for its summer campaign for Sudan. If you haven't written these letters, now would be a great time! If you need some inspiration, check out this Amnesty Magazine article or this video about refugees in Chad.
(Oops! I think it's great that Amnesty is uploading campaign videos to the web, but this one is missing some subtitles. This page has a shorter version that fills in some gaps.)