Brief Summary
Firefox have this bar in the bottom of the "address window" that lets you choose where the search will happen, if you want to search a different place then the default search engine

Details:
To some extend its very similar to "Bangs" but not all search engines support that. and this is way more intuative for people who dont know about bangs
I think this could be super usefull, especially if you could configure what sites you have as options. example.. github, youtube, wiki. whatever fits your work/freetime needs
My only problem with Firefoxs implementation is that when you actually click the icon of where you want to search, it inserts kinda like a bang instead of taking you directly there, so you still have to press enter after.
I dont see a need for that extra step.