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 type
ant PhyloWidgetat the command prompt. I got a couple of warnings about missing .keystore files (something to do with signing the applet), but otherwise things seemed to work.
The applet has a URL API, which makes it easy to view trees. For example, try this link to view the Frost et al. amphibian tree (doi:10.1206/0003-0090(2006)297[0001:TATOL]2.0.CO;2).