Ipe is a nice power tool - I learned to use it in grad school and have used it to draw technical illustrations that are essentially polished sketches. For example Figures 1 through 8 in this paper: https://users.cs.duke.edu/~pankaj/publications/papers/flood-...
Other people like TikZ, but the kinds of illustrations I gravitate towards rarely have a neat exact compass and straightedge feel that lends itself to coding in TikZ (e.g. Figure 4). Then for certain figures I have used Ipe as an intermediate language similar to SVG, where I would write a Python program to produce some precise drawing that I could then tweak by hand in the Ipe GUI (e.g. Figure 2).