Gnome Do is a cross-platform launcher written in C#. It is looks very similar to Quicksilver. Source code is hosted on Launchpad.
Kupfer is an another launcher. It includes large number of builtin plugins. It is also written in Python and extending it with custom plugins is easier and simpler than extending other launchers. There is an detailed PluginAPI documentation which includes instructions how to write plugins. Source code is hosted on Gnome Git.
There are three main building blocks in Kupfer: Leafs, Sources and Actions. Description from the documentation:
A Leaf is an object, it represents a program or a file, or a text or something else. Every type of Leaf has different possibilities, and you can define new Leaves. Example: a
FileLeafrepresents a file on the disk.
A Source produces a collection of Leaves, so it makes Kupfer know about new objects. For example, it can provide all the FileLeaves for a particular folder.
An Action is the part where something happens, an action is applied to a Leaf, and something happens. For example, Open can be an action that works with all
Mutate is a simple launcher inspired by Alfred. It is written in C++ and Qt. An interesting feature is that you can include scripts in arbitrary languages like python or bash. Scripts are specified in an INI config. Eeach item includes name, script file, icon and whether it needs arguments. When the given item is invoked in command input then the script file is executed.
Launchy is a cross-platform launcher implemented in C++ and Qt. It looks like it is currently an abandoned project. It features plugin architecture. Basic plugins include calculator, web search. It seemed that Launchy does not allow to specify additional action on an item. Source code is hosted on SourceForge.