"SendKeys" equivalent in Linux

"SendKeys" equivalent in Linux, also the old discussion about How can I modify a keyboard event before it reaches an app?

1. xmacroplay
   The XMacro package contains two simple, C++ programs (xmacrorec and xmacroplay) for recording and replaying keyboard and mouse events on an X server.
   
2. xremote
   xremote is a simple tool that lets you grab the mouse and keyboard of another machine and control them with your local mouse and keyboard. It creates a form of remote control of other machines.
   

1. wmctrl
   The wmctrl program is a UNIX/Linux command line tool to interact with an EWMH/NetWM compatible X Window Manager.
   
2. GKrellM
   GKrellM is a single process stack of system monitors which supports applying themes to match its appearance to your window manager, Gtk, or any other theme.