Knotter is a free and open source customizable interlace designer.

Interlace patterns are a kind of design historically used as a
decorations in many places and by different cultures. Some examples
are Celtic knotworks and Islamic interlaces. Knotter aims to allow its
user to intuitively design such patterns and to provide easy ways to
integrate the result in external general-purpose graphic software for
this purpose designs created within Knotter can be saved in a custom
Human-Readable format and exported as scalable vector graphics and in
a wide number of raster image formats.