LingTree is a tool that lets you easily describe a linguistic tree and produce a graphic image of it. You can use it for syntactic, morphological, and phonological tree diagrams. It outputs in PNG and SVG formats. With the SVG format, you can also use a tool like Inkscape to add extra features to it, such as curved lines and arrows.
You can then insert these LingTree-produced tree diagrams in linguistic documents or web pages.