At Admin Panel -> Themes -> Custom Pages, you have the option to add custom pages to your site. Custom pages are very much like custom templates, but they apply across all your themes. Unlike custom templates, they can't be overlayed on a specific other page (they're for new pages, not alternate versions of existing pages).

Fill in the title and content to create the custom page. You can use HTML and toplists (Admin -> Themes -> Toplist Generator) in the content, as well as all the usual template variables.

Once you've created a custom page, it'll give you the linking code you can use to reference it. Normally you would want to link to it in your wrapper template, Admin Panel -> Themes -> Manage Templates -> "Wrapper".

For Versions 9.0+

As of WSN 9, you can choose to edit in either plain text or WYSIWYG mode -- plain text is safer, WYSIWYG will usually work but could mangle some syntax if including toplists etc. You can also now specify a custom URL for the page if you have URL rewriting on (if you want URL rewriting on, go to Admin -> Settings -> SEO). There's also an option to include the page in the main menu... if you select this, the page will be automatically added to the main menu which appears at the top of your site and will appear in your menu manager at Admin -> Customizations -> Menu Manager.