What are the alternatives to OS X's Spotlight?

Unity Dash

From 11.04 to 17.04 (Unity), Ubuntu had an even better solution built in: The Dash!

With the dash you can do all kind of cool stuff, such as:

• Search through your files
• Find installed and available applications
• Run commands
• There will be many more features in the future, as the dash is extendible through so called "lenses". These will allow you, for example, search Ask Ubuntu right from your desktop.

Just hit Super to open it in 11.04!

For more information, check out this link.

Cardapio

Cardapio is exactly what I was looking for. You'll have to install TrackerDownload tracker, start it, then enable the "File Search" plugin from Cardapio.

Deskbar

Deskbar is probably the closest application, as far as the user interface goes, to spotlight.

Click here to install.

It has a number of backends, allowing you to search for files, launch applications, search the web, and a lot more.