On the topic of Mesopotamian religion: http://oracc.museum.upenn.edu/amgg/ has a nice reference list for Mesopotamian mythology and the attached website has a large number of ritual texts and a few mythological texts. the Electronic Text Corpus of Sumerian Literature is a good database of Sumerian literature including mythological texts.

On Mesoamerican myth: A good Internet clearinghouse for all things Mesoamerica is http://www.famsi.org/; it takes a bit of navigating to find specifically mythological material but it is a good collection of research papers, drawings of artworks, and so on.