If you want to create diagrams from textual descriptions, https://kroki.io/ is very versatile and supports several different markup languages including C4.

Alternatively, if you want to stick to ASCII output, https://arthursonzogni.com/Diagon/#Sequence supports several formats (see the dropdown) which can pipe into something like Typogram (https://google.github.io/typograms/) for more beautiful output.

For example, input:

  Renderer -> Browser: BeginNavigation()
  Browser -> Network: URLRequest()
  Browser <- Network: URLResponse()
  Renderer <- Browser: CommitNavigation()
  Renderer -> Browser: DidCommitNavigation()

will output the following sequence diagram:

  .--------.            .-------.     .-------.
  |Renderer|            |Browser|     |Network|
  '--------'            '-------'     '-------'
      |                     |             |    
      |  BeginNavigation()  |             |    
      |-------------------->|             |    
      |                     |             |    
      |                     |URLRequest() |    
      |                     |------------>|    
      |                     |             |    
      |                     |URLResponse()|    
      |                     |<------------|    
      |                     |             |    
      | CommitNavigation()  |             |    
      |<--------------------|             |    
      |                     |             |    
      |DidCommitNavigation()|             |    
      |-------------------->|             |    
  .--------.            .-------.     .-------.
  |Renderer|            |Browser|     |Network|
  '--------'            '-------'     '-------'

and then you can perform further edits using something like https://asciiflow.com/ (web, free) or https://ivanceras.github.io/bob-editor/ (web, free) or https://monodraw.helftone.com/ (Mac only, proprietary) as mentioned in other comments from https://news.ycombinator.com/item?id=37040883.