Specview is a Java applet. This means you have to have a java virtual machine active within your browser. A common way to get this is to install a plugin, available, e.g., from java.sun.com, or by installing a package like openjdk-?-jre on Debian-derived systems (where ? corresponds to some version).

Specview is a product of the Space Telescope Science Institute, which is operated by AURA for NASA. Its source code is available under a BSD-style licence from its sourceforge page

If you see all kinds of security warnings, see STScI's explanations on them.