Earthly Affair is a wonderful resource for earth-friendly invitations.  Although the company specializes in wedding invitations, I can see myself using these for birthday party or formal dinner invitations.  My favorite design is the Alcazar design shown below in a the fuschia and brown.

Earthly Affair offers two earth-friendly papers for their invitations: 100% post consumer waste or partially recycled with the rest of the paper coming from FSC (Forest Stewardship Certified) trees.  For more info on their earth-friendly practices or to order these beautiful invitations click here.