From DT Online
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 and exported as scalable vector graphics and in a wide number of raster image formats.
DT Online strongly advises that the Tutorial be followed first to achieve a good result.