Ulauncher!

Listary is an excellent enhancement utility for list controls used everywhere: Windows Explorer, Task Manager, Registry Editor, and innumerable third-party applications.It searches folders, files or anything else as-you-type. Autocomplete, masks, hotkeys, regular expressions (pro version only) are good time-saving features.

Quicksilver is a fast and free Mac OS X productivity application that gives you the power to control your Mac quickly and elegantly. Quicksilver learns your habits, making your everyday chores simple and efficient.

GNOME Do allows you to quickly search for many items present on your desktop or the web, and perform useful actions on those items.

Cerebro is an open source and free electron-based productivity software alternative to Alfred and Spotlight. It is focused on speed and good UI and UX. In most cases you don't even need to open another application, because you can see results right in Cerebro, like google maps, IMDV movies or contact details.

Zazu is a new open source productivity app, that allows developers to be in the drivers seat. Unlike other solutions, there is no built-in integrations, so every plugin has the full scope of what Zazu has to offer.

Google Quick Search Box is an open source search box that allows you to search data on your computer and across the web.

Krunner is a multi-function command line menu for the KDE Plasma Desktop.At times, it almost seems as you could manage all your computing from Krunner. Among other things, you can launch commands, open websites using the abbreviations defined in System Settings > Shortcuts > Web Shortcuts, send emails, convert units of measurements, open particular system settings, and view the date and time.

LaunchIt is an Android application similar to the likes of QuickSilver for Mac, Launchy for Windows, and GnomeDo for Linux. It allows you to quickly find information on your Android device, such as Applications, Bookmarks, Contacts, Media and more.