What is ngspice?
Simulation Program with Integrated Circuit Emphasis. NGSPICE is an open source mixed-signal circuit simulator. It is freely available for use in Linux and Windows. It is recommended to use Linux for NGSPICE. NGSPICE requires you to describe your circuit as a netlist. A netlist is defined as a set of circuit components and their interconnections.
Why install from source?
Because the most recent code has some nasty bug fixed. If not, you can fix the bugs in the code yourself, build and install the freshly built binaries.
Well, there is this one bug I found, when you plot multiple lines like in transistor output characteristics, you get the lines joining back to the origin after the last point as seen below.