Bang Search од TheNamlessGuy

Allows you to use !bangs to search the web better. For example, using the default engines, searching for:
hello world !yt
in your search bar (/URL bar) will redirect you to a YouTube search of "hello world".

The bang can go anywhere in the query:
!yt hello world
hello world !yt
hello!yt world
and so on!

You can also add your own engines (and remove default ones) in the option page.

When you don't specify a bang, the addon either:
* opens up a page, letting you choose which of your engines you want to use (default)
* selects a predefined default bang

All of this is handled locally in your browser - no request goes externally (until you actually use a bang/select a search engine).

How does it work? An addon isn't really allowed to interact with a search request in the state between the user pressing enter, and the request getting reformatted to the search engine URL.
However, by adding its own search engine which points to an invalid URL (bangs.invalid - which can never actually exist, since .invalid is flagged as never being a real TLD), it can intercept its own spawned requests to said invalid URL, and perform the actions and calculations needed from there on.