PLink is a full featured graphical editor for knot and link projections, using the cross-platform GUI toolkit Tk. The primary focus is on piecewise-linear link projections, but it also supports a "smooth mode" and can export images in PostScript, PDF, SVG, and TikZ formats. See the PLink home page for complete details.
You can browse the source code here.
Copyright 2008-present by Marc Culler, Nathan Dunfield, and others.
All parts of this package are released under the GNU General Public License, version 2 or (at your discretion) any later version as published by the Free Software Foundation.
The development of this program was partially supported by the US National Science Foundation under grants DMS0608567, DMS0504975, DMS0204142, and others.