@Ivaylo Valkov Wait, an even easier way to do it, just right click any search input form and click "Add a Keyword for this Search..." and it will do all of the necessary work for you.

I didn't know that, very cool!

Anyway.

I use this feature a lot, for example a have a quick search bookmark for Google Translate.

I bookmarked: http://translate.google.com/translate_t?hl=en&sl=auto&tl=en&js=y&text=%s

With the keyword: trans

When I need to translate a phrase like "Kühles Kind!", a language I don't understand, I can simply enter "trans Kühles Kind!" and go directly to the Detect Language to English Google translation.

So the search extensions keywords aren't the only way to get the functionality, like TildeHash.com doesn't have a search extension, but I have a quick search bookmark for it, myself.