Here are some of my favourite tools:
IBP – Axandra’s Internet Business Promoter. I use it for SEO and link building. I can see how search engines view my sites, compare my sites rankings and inbound links against competitors, submit my sites to directories, and I use the Arelis link management system to find and track sites I have approached for links, and to actually build my link pages. Very useful for SEO.
PPCWebSpy – Brad Callen’s Firefox plugin that lets you spy on competitor AdWords campaigns and keywords.
WordPress.org – WordPress is the most popular free blogging software. I use it for Geekmum. WordPress.org requires your own hosting, but you can customise it as much as you like. WordPress.com provides hosting, but you can’t put affiliate links on your site then.
Drupal.org – Drupal is a free open-source content management system. Although WordPress also allows you to have static menu pages, events, and plugins for galleries, Drupal is much more flexible, with user management and profiles suitable for a community site, and the ability to define your own data types. For example I’ve used Drupal to create MyOnlineShops, with separate data types for shops, coupons and sales.