Having simple knotwork that can fill a space is very useful, and it can be very simple (but time consuming!) to do. If you only want to learn a little knotwork, this is one I definitely recommend!
Aon Celtic, a site created by the artist Cari Buziak, is full of other useful stuff. I highly recommend her free downloadable dot paper. Making grids from scratch or modifying graph paper is probably the most tedious part of knotwork!