Firefox only (Windows/Mac/Linux): The Omnibar extension makes Firefox's awesomebar act like Google Chrome's omnibar, collapsing the address bar and search into one single text box.
We've mentioned Omnibar in passing in a couple of posts, namely our guide to turning Firefox into a Google Chrome clone and our post detailing how to use the Firefox 4.0 theme, but it's a useful extension whether or not you're trying to make Firefox look like another browser. You can still switch your default search engines, you can still use keyword bookmarking, and the extension features special shortcuts for searching non-default engines (try your query with @y, for example, to search Yahoo).
Omnibar is a free download, works wherever Firefox does. Thanks Hampus!
Omnibar [Firefox Add-ons]