Adblock Plus - disabled by default and enable per site

I am assuming that you are asking about Adblock Plus. I cannot answer your question for other extensions, some of them implemented the Adblock Plus filter syntax in a slightly broken way.

Yes, running Adblock Plus in a whitelist mode is possible. It is somewhat awkward however, the user interface support for this feature is still very lacking. First of all you need to add the custom filter @@*$document that will have the effect of disabling Adblock Plus everywhere.

To add this filter in Firefox:

• Click the Adblock Plus icon.
• Choose "Filter Preferences" from the menu (you can also press Ctrl+Shift+F).
• Switch to "Custom filters" tab.
• If you don't have any filter group yet, click "Add filter group". Otherwise just select it.
• Click "Add filter" button on the right and enter this filter.

To add this filter in Chrome or Opera:

• Right-click the Adblock Plus icon.
• Choose "Options" from the menu.
• Switch to "Add your own filters" tab.
• Enter the filter into the text field and click "Add filter".

The tricky part is updating the filter whenever you want to allow blocking on a particular website. Currently that's significantly simpler in Firefox:

• When on the website, click the Adblock Plus icon.
• Choose "Open blockable items" from the menu (you can also press Ctrl+Shift+V).
• There will be only one entry, right-click it.
• Choose "Disable this filter on example.com" where example.com is the site you are on.
• Click the cross symbol in the upper-right corner of the blockable items list or press Ctrl+Shift+V again to close blockable items.

This will automatically change the filter you've added into something like @@*$document,domain=~example.com.

Now if you are on Chrome or Opera, there is currently no way to do that automatically - you will have to go into Adblock Plus options and edit the filter manually. That's actually easier if you click the "Edit filters as raw text" at the bottom of the "Add your own filters" tab. If you want to allow blocking on multiple domains the filter has too look like this: @@*$document,domain=~example.com|~example.info|~foo.example.org.

Disclaimer: I am the lead developer of Adblock Plus.