It reminds me a little of another piece of open source software, Notepad++ - not flashy but very powerful.
Hint for anyone wanting to use it on Windows: The quickest way to get it going is to download the "Microsoft Windows portable binary" because it contains everything needed.
Thanks Jim. ThinkingExpress seems surprisingly feature-rich despite its plain look.
As I haven't heard it mentioned before, first I took the precaution of checking the executable at VirusTotal, and 42 out of the 42 anti-virus engines there found it clean.