Xschem is a schematic capture program, it allows creation of
hierarchical representation of circuits with a top down approach . By
focusing on interfaces, hierarchy and instance properties a complex
system can be described in terms of simpler building blocks. 
A VHDL or Verilog or Spice netlist can be generated from the drawn
schematic, allowing the simulation of the circuit.
Key feature of the program is its drawing engine written in C and using
directly the Xlib drawing primitives; this gives very good speed
performance, even on very big circuits. 
The user interface is built with the Tcl-Tk toolkit, tcl is
also the extension language used.  

- hierarchical schematic drawings, no limits on size 
- any object in the schematic can have any sort of properties 
  (generic in VHDL, parameters in Spice or Verilog) 
- new Spice/Verilog primitives can be created, and the netlist format
  can be defined by the user 
- back annotation for variety of device parameters in a hierarchical
  fashion when combined with ngspice simulator.
- built-in waveform viewer in the schematic with back-annotation
  dynamically linken with the "b" cursor
- ngspice integration and other simulator as well
- TCP interface to BeSpice wave
- tcl extension language allows the creation of scripts. 
  any user command in the drawing window has an associated tcl comand
- VHDL / Verilog / Spice netlist, ready for simulation 
- Behavioral VHDL / Verilog code can be embedded as one of the
  properties of the schematic block.