TikZ 3.0 introduced the pic element, which works like a combination of node and tikzpicture features. Here we create a pic for a computer, which can be used several times with modified appearance.
This example was written by Elke Schubert on TeXwelt.de. http://texwelt.de/wissen/fragen/3536/wie-erstelle-ich-neue-shapes-in-tikz

Edit and compile if you like:
% Computer diagram% Author: Elke Schubert\documentclass[tikz,border=10pt]{standalone}\usetikzlibrary{fit}\tikzset{comp/.style = {minimum width = 8cm,minimum height = 4.5cm,text width = 8cm,inner sep = 0pt,text = green,align = center,font = \Huge,transform shape,thick},monitor/.style = {draw = none, xscale = 18/16, yscale = 11/9},display/.style = {shading = axis, left color = black!60, right color = black},ut/.style = {fill = gray}}\tikzset{computer/.pic = {% screen (with border)\node(-m) [comp, pic actions, monitor]{\phantom{\parbox{\linewidth}{\tikzpictext}}};% display (without border)\node[comp, pic actions, display] {\tikzpictext};\begin{scope}[x = (-m.east), y = (-m.north)]% filling the lower part\path[pic actions, draw = none]([yshift=2\pgflinewidth]-0.1,-1) -- (-0.1,-1.3) -- (-1,-1.3) --(-1,-2.4) -- (1,-2.4) -- (1,-1.3) -- (0.1,-1.3) --([yshift=2\pgflinewidth]0.1,-1);% filling the border of the lower part\path[ut](-1,-2.4) rectangle (1,-1.3)(-0.9,-1.4) -- (-0.7,-2.3) -- (0.7,-2.3) -- (0.9,-1.4) -- cycle;% drawing the frame of the whole computer\path[pic actions, fill = none](-1,1) -- (-1,-1) -- (-0.1,-1) -- (-0.1,-1.3) -- (-1,-1.3) --(-1,-2.4) coordinate(sw)coordinate[pos=0.5] (-b west) --
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