Multiplatform tools
This is a short list of free software tools frequently used under more than one platform or operating system to work with Tikiwiki (for instance, GNU/Linux, Windows, Mac, ...):

1.1.1. Many app.s from KDE

KDE4 + KDE4Windows + KDE4Mac: KDE is an international technology team that creates Free Software for desktop and portable computing. Among KDE's products are a modern desktop system for Linux and UNIX platforms, comprehensive office productivity and groupware suites and hundreds of software titles in many categories including Internet and web applications, multimedia, entertainment, educational, graphics and software development. KDE software is translated into more than 60 languages and is built with ease of use and modern accessibility principles in mind. KDE4's full-featured applications run natively on Linux, BSD, Solaris, Windows and Mac OS X.
http://kde.org - http://windows.kde.org - http://mac.kde.org

1.1.2. Text processing in local machine

Web developer toolbar: Adds a menu and a toolbar with various web developer tools.
https://addons.mozilla.org/firefox/addon/60 - http://chrispederick.com/work/web-developer/

ColorZilla: A collection of tools, including a Color Picker similar to the one found in Photoshop, an Eyedropper that helps to get a color reading from anywhere in the Desktop.
https://addons.mozilla.org/firefox/addon/271

Document Map: Displays the current page's heading structure in the sidebar, allowing rapid navigation between sections.
https://addons.mozilla.org/firefox/addon/475

FireBUG: DOM inspector, debugger, just about anything a dev could want (apart from a colour picker!)
https://addons.mozilla.org/firefox/addon/1843

FreeMind: Utility to draw mind maps. See PluginMindmap
http://freemind.sourceforge.net/

Eclipse: Integrated Development Environment (IDE). A project aiming to provide a universal toolset for development. Open Source IDE, mostly provided in Java, but the development language is independent and allows editing code in many languages, including PHP. See Eclipse
http://www.eclipse.org

NetBeans: Integrated Development Environment (IDE) Runs on Windows, Linux, Mac OS X and Solaris. NetBeans IDE is open-source and free. Ajax | C/C++ | Databases | Debugger | Desktop | Editor | Groovy | GUI Builder | Java EE | JavaFX | Java ME | Java SE | JavaScript | Mobile | PHP | Profiler | Python | Refactor | REST | Rich Client Platform | Ruby | SOA | SOAP | UML | Web | WSDL | XML
http://www.netbeans.org

PhpMyAdmin: A tool written in PHP intended to handle the administration of MySQL over the web.
http://www.phpmyadmin.net/

XAMPP: An easy to install Apache Distribution for Linux, Windows, Mac OS X, and Solaris. The package includes the Apache web server, MySQL, PHP and Perl. XAMPP is really very easy to install and to use - just download, extract and start.
http://www.apachefriends.org/en/xampp.html

Aptana: Available either as a stand-alone app, of as an Eclipse plugin - specific web development environment, including HTML, CSS, JavaScript support and FTP synchronisation. http://www.aptana.com

Gimp: The GNU Image Manipulation Program, for X Windows systems, ported also to other platforms. Official site, including download area, documentation, FAQs, and a plugin registry.
http://www.gimp.org/

Inkscape: An Open Source vector graphics editor, with capabilities similar to Illustrator, CorelDraw, or Xara X, using the W3C standard Scalable Vector Graphics (SVG) file format. Inkscape supports many advanced SVG features (markers, clones, alpha blending, etc.) and great care is taken in designing a streamlined interface. It is very easy to edit nodes, perform complex path operations, trace bitmaps and much more. We also aim to maintain a thriving user and developer community by using open, community-oriented development.
http://www.inkscape.org

Pidgin (ex-Gaim): An instant messaging client, which can handle multiple protocols, including AIM, ICQ, IRC, MSN, Yahoo!, Jabber, and Gadu-Gadu. [Windows, Unix]
http://pidgin.im

Freeware (but not free software) tools

Wink: A tutorial and presentation authoring software.
http://www.debugmode.com/wink/

(You might be interested in checking also the Linux tools, Windows tools or Mac Tools, specific of those platforms).