It really depends on the type of software we are talking about. Something proprietary like Windows 7 or Photoshop? Don't even think about it. Extremely illegal and could mean trouble for a very long time. These billion dollar corporations take intellectual property very seriously. However, if the software is under the "GNU" license, the source code is most likely available and your involvement in development, very encouraged. These are things like GIMP, or most Linux Distributions. A good site to find more information is SourceForge where many developers can put open source software on display for review and use.