Blogosphere gurus Instapundit (www.instapundit.com) and Truth Laid Bear (see link in title) with aid from Technorati (http://technorati.com/tag/flood%20aid) are coordinating an effort to have bloggers cite a relief agency to support by tomorrow. As a sporadic attender of Grace Nazarene in Clarksville, I recommend Nazarene Compassiate Ministries (http://www.ncm.org/news_katrina_8_31.html). Having said that, I have had previous experiences with Habitat for Humanity and World Vision and would be glad to recommend either of them as well. The Instapundit and Truth Laid Bear sites have many other excellent sites for those less comfortable giving to a Protestant charity.
P. S. Free Money Finance is recommending Samaritan's Purse, which basically grew out of Billy Graham's ministry.