如果只需要简单的功能的话,推荐 CS Academy
如果需要比较复杂的功能,推荐 Graphviz Online
比如自动机可以这么画:
digraph G { start -> start [label="0"] start -> 1 [label="1"] 1 -> 2 [label="0"] 2 -> 3 [label="1"] 3 -> 4 [label="1"] 3 -> 2 [label="0"] 2 -> start [label="0"] 1 -> 1 [label="1"] start [shape=doublecircle] 4 [style=filled] }