FreeTikZ is a web-based tool to convert hand-drawn diagrams into tikz code, tailored to graphical languages for monoidal categories. It is meant as a quick way to generate tikz, that you can then fine-tune by hand. You will need freetikz.sty and standalone to compile the latex code.

turns into which compiles to

Click here to run.

FreeTikZ is open source. You can simply download the source. That also means you can run it offline. Tinker to your hearts content to make it fit your needs. Please do email the developer if you make improvements.