What software tools, if any, which can count the number of syllables on each line, to aid in the writing of English-language poetry? What features and pros and cons exist for each?
This is not something strange. Almost every programming language can analyze text to capture such data. It will be a one-file script. Here is some online applications:
Since the process is simple and straightforward, the features are not very different; just analyzing the text. There should be lots of (simple) desktop softwares too.