I suppose I could've explained it... It's ideal for keyboard-heavy users.
Instead of typing a search term into the search box and Ctrl+Arrow-ing to the search engine I want to use, with YubNub (in Firefox and Chrome, maybe IE?) you type in a command for what you want to do.
"g FDR" does a Google search of "FDR".
b Bing, y Yahoo, yt YouTube, CNN, ESPN, IMDB, and on and on.
There's even some slightly stronger commands:
"tr Chi Eng 晚安" uses Google Translate from Chinese to English, telling me "晚安" means "good night".
Yeah, I mostly use the "g" and "yt" commands, but I love the flexibility without having to arrange search engines. I just remember a few letters/commands. For a long time I'd resorted to typing "site:imdb.com" as my first search term into a Google search box. No more.
Also, you can define your own shortcuts! On a whim, I pointed XBLA to the Xbox Live marketplace at Xbox.com, and it works wonderfully. So now I can just type "xbla Trenched" and get to the page so I can buy the game with a click.
As sesqu said, this features is built into firefox. I only have a window-wide url bar on my firefox and extensively use these "keyworded" bookmarks.
So in my url bar I can "g foo" to search "foo" using google, "wp foo" for wikipedia fr, "wpe foo" for wikipedia english, "imdb foo" for imdb, "yt foo" for youtube, "dm foo" for dailymotion, "gdv foo" to load the document which url is "foo" into google docs viewer, "tw" take me to twitter, "fb" to facebook, "hn" to Hacker News, "mail" to gmail, "reader" to google reader, "in" to linked in... I don't have to all list in my head, it's mostly a finger habits now.
What is really cool since firefox has the awesomebar is that it is aware of this and display the actual url so when i type "imdb foo bar" i see "http://www.imdb.com/find?s=all&q=foo+bar at the top of the proposed url.
This is really handy for people like me who mostly use their keyboard: this powerful firefox feature is just a ctrl+L away when I'm already in firefox :-).
This functionality is actually built into Firefox and Chrome, and I'd be shocked if at least Opera didn't have it too. It's the "keyword" field in search engine settings.
I believe multiple parameters are also allowed, but not having used them, can't vouch.