Rofi started as clone of simpleswitcher, written by Sean Pringle a popup window switcher roughly based on superswitcher. Simpleswitcher laid the foundations and therefor Sean Pringle deserves most of the credit for this tool. Rofi, renamed as it lost the simple property, has been extended with extra features, like a run-dialog, ssh-launcher and can act as a drop-in dmenu replacement, making it a very versatile tool. Rofi, like dmenu, will provide the user with a textual list of options where one or more can be selected. This can either be, running an application, selecting a window or options provided by an external script.
Very fast, lightweight, and configurable. So fast it doesn't even need to run in the background like other launchers.
good overall. has performance issues though IIRC due to indexing or caching