Nxt Source Code
From Nxt Wiki
Contents
Current Version: 1.3.4
The currently released version is 1.3.4.
Version 0.7.6
The NXT source code for version 0.7.6 has been released on March 1st, 2014.
For more information about this released source code, see here.
Version 0.4.7
The NXT source code for version 0.4.7 has been released on January 3rd, 2014.
For more information about this released source code, see here.
Flaw report
You may also be interested in the Nxt source code flaw report thread on bitcointalk.org.
Source code for any version
You can see the source code for any version of Nxt by running it through a Java decompiler such as JD.