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AIX Security Toolkit
The original design of the unix operating system offered only basic security. AIX has provided features which have significantly improved the security potential of the operating system. The building blocks exist, but because AIX maintains compliance with POSIX and other standards, implementing a secure system is often difficult (for example, many security features are extensions to basic unix functionality). The toolkit was developed to meet this challenge on versions 4 and 5 of AIX.
Read accompanying article: Securing IBM's AIX
The toolkit is provided "AS IS", without support. Also read the ROOTVG.NET disclaimer.
If you want to contribute to the AIX Security Toolkit or have suggestions for changes or new features you can post them on the AIX Security Forum. If you have any AIX related Security or Toolkit questions you can also post them there.
Read more and download at ROOTVG.NET: AIX Security Toolkit (Latest update May 2003).
Contribution of Simon Taylor

nmon - free tool to analyze AIX performance
The nmon (and nmon analyser) tool is helpful in presenting all the important performance tuning information on one screen and dynamically updating it. The tool works on any dumb screen, telnet session, or even dial-up line. In addition, the tool is very efficient. It does not consume many CPU cycles, usually below 2%. On newer machines, CPU usage is well below 1%. It collects a lot more performance information than topas. 
The nmon tool can also capture the same data to a text file for later analysis and graphing for reports (spreadsheet format .csv).
The nmon tool was developed to support benchmarks and performance tuning for IBM internal use but by popular demand is given away to deserving friends.
Download at http://www-106.ibm.com/developerworks/eserver/articles/analyze_aix/

Doxygen is a documentation system for C++, Java, IDL (Corba, Microsoft and KDE-DCOP flavors) and C.
It can help you in three ways:
1. It can generate an on-line documentation browser (in HTML) and/or an off-line reference manual (in ) from a set of documented source files.
There is also support for generating output in RTF (MS-Word), PostScript, hyperlinked PDF, compressed HTML, and Unix man pages. The documentation is extracted directly from the sources, which makes it much easier to keep the documentation consistent with the source code.
2. Doxygen can be configured to extract the code structure from undocumented source files. This can be very useful to quickly find your way in large source distributions. The relations between the various elements are be visualized by means of include dependency graphs, inheritance diagrams, and collaboration diagrams, which are all generated automatically.
3. You can even `abuse' doxygen for creating normal documentation.
Doxygen is developed under Linux, but I created smit installable binairies for AIX 4.3.3.
DownloadLocationSW: http://www.stack.nl/~dimitri/doxygen/index.html

VIM (vi improved by Bram Moolenaar)
VIM adds many of the features that you would expect in an editor: Unlimited undo, syntax coloring, split windows, visual selection, graphical user interface (read: menus, mouse control, scrollbars, text selection), and much much more.
Contribution of Sven Guckes http://www.vim.org

place and list utility
place utility for analysing fragmentation of your files and filesystems.
list utility
which will list the files and data that can be recovered with the Analyse (undelete) utility from Compunix.
You can also take a look their Filesystem Utilities: reduce or shrink filesystems, defragment your files and filesystems, and recover deleted files.
Contribution of Compunix

Storix Backup Administrator for AIX (SBA)
Storix Backup Administrator (SBA) can backup the local system or all of your RS6000s and SP nodes in a single step!
Contribution of Storix http://www.storix.com

pdfcrypt, txt2pdf and jpg2pdf 
pdfcrypt is a very flexible and powerful program. pdfcrypt allows you to encrypt a PDF (40 bits and 128 bits), set permissions, add user and owner password. You can freely redistribute pdfcrypt tool in its unregistered archive format (zip or tar.gz):
txt2pdf is a very flexible PERL5 program that allows you to convert all your old .TXT files to PDF format.
jpg2pdf is a very flexible and powerful PERL5 program that can convert a collection of jpeg images into a unique PDF.
The perl source code of these products + an AIX executable version you can find at
Contribution of Sanface Software http://www.sanface.com

monitor and mpage
AIX monitor and mpage. Open Source software developed by and/or maintained by MESA.
Contribution of Mesa Consulting BV http://www.mesa.nl

* ROOTVG.NET hosts mostly links to the download location of the software.