Similar to Firefox's extension NoScript. Blocks all external scripts by default. This prevents most ads from showing up and also increases security. Tested under Opera 12.17

For this script to work on secure pages, you will need to enable this setting : opera:config#UserPrefs|UserJavaScriptonHTTPS

Inline and same domain Javascript is allowed. All the rest is blocked by default. You can add exceptions via a menu for each site.
Each domain (ex: forums.example.com, www.anothersite.org, etc...) gets its own list of allowed external scripts.
This script is a complement to the native opera feature which allows you to disable/enable javascript globally and per-site via Manage Site Preferences.

The most secure setting is to :

1. Disable Javascript globally. ( Tools > Preferences > Advanced > Content > Uncheck Enable Javascript )
2. For each site where you want Javascript to work. Enable it locally. ( Right-Click > Edit Site Preferences > Scripting > Check Enable Javascript )
3. Reload the page
4. External Script Blocker will then activate, and you'll be able to choose which domains to allow.