OpenLP: Open Source video projection system for churches. We've have found it to be robust enough for all our needs. Requires powerpoint, which is used in the background.
Inkscape: Open Source vector graphics package using SVG. Similar to Adobe Freehand. We've used this to design most of our church flyers and literature. Will export to EPS but not import in the current Beta version.
The Gimp: Open Source picture editing, similar to Adobe Photoshop.
XnView: Will import 400+ different graphic formats. Great for conversion between formats. Contains some basic editing functions and filters.
The Online Bible: Less useful since the advent of the web but still helpful occasionally.