Greg Jordan and Bill Piel have released PhyloWidget, a Java applet for viewing phylogenetic trees. It's very slick, with some nice visual effects courtesy of Processing. PhyloWidget is open source, with code hosted by Google code. I'm a C++ luddite, so it took me a few moments to figure out how to build the applet, but it's simple enough, just typeant PhyloWidgetat the command prompt.