The current help information as it would be printed at the command line.

§1. Running Inweb with -help currently produces the following summary:

inweb: a tool for literate programming

Usage: inweb WEB OPTIONS RANGE

WEB must be a directory holding a literate program (a 'web')

The legal RANGEs are:
   all: complete web (the default if no TARGETS set)
   P: all preliminaries
   1: Chapter 1 (and so on)
   A: Appendix A (and so on, up to Appendix O)
   3/eg: section with abbreviated name "3/eg" (and so on)
You can also, or instead, specify:
   index: to weave an HTML page indexing the project
   chapters: to weave all chapters as individual documents
   sections: ditto with sections

-import-from X           specify that imported modules are at pathname X
-verbose                 explain what inweb is doing (default is -no-verbose)

for locating programming language definitions:
  -read-language X       read language definition from file X
  -read-languages X      read all language definitions in path X
  -show-languages        list programming languages supported by Inweb
  -test-language X       test language X on...
  -test-language-on X    ...the code in the file X

for analysing a web:
  -advance-build         increment daily build code for the web
  -advance-build-file X  increment daily build code in file X
  -catalog               same as '-catalogue'
  -catalogue             list the sections in the web
  -functions             catalogue the functions in the web
  -gitignore X           write a .gitignore file for this web and store it in X
  -makefile X            write a makefile for this web and store it in X
  -platform X            use platform X (e.g. 'windows') when making e.g. makefiles
  -prototype X           translate makefile from prototype X
  -scan                  scan the web
  -structures            catalogue the structures in the web
  -write-me X            write a read-me file following instructions in file X

for weaving a web:
  -breadcrumb X          use the text X as a breadcrumb in overhead navigation
  -navigation X          use the file X as a column of navigation links
  -open                  weave then open woven file
  -weave                 weave the web into human-readable form
  -weave-as X            set weave pattern to X (default is 'HTML')
  -weave-into X          weave, but into directory X
  -weave-tag X           weave, but only using material tagged as X
  -weave-to X            weave, but to filename X (for single files only)

for tangling a web:
  -ctags-to X            tangle, but write Universal Ctags file to X not to 'tags'
  -no-ctags              don't write a Universal Ctags file when tangling (default is -ctags)
  -tangle                tangle the web into machine-compilable form
  -tangle-to X           tangle, but to filename X

for dealing with colonies of webs together:
  -colony X              use the file X as a list of webs in this colony
  -member X              use member X from the colony as our web

-at X                    specify that this tool is installed at X
-crash                   intentionally crash on internal errors, for backtracing (default is -no-crash)
-fixtime                 pretend the time is 11 a.m. on 28 March 2016 for testing (default is -no-fixtime)
-help                    print this help information
-locale X                set locales as 'L=E', L being shell or console, E platform, utf-8 or iso-latin1
-log X                   write the debugging log to include diagnostics on X
-version                 print out version number

§2. Running Inweb with -show-languages currently produces the following list of programming languages for which support is provided in the standard distribution:

I can see the following programming language definitions:

ACME: The ACME assembly language for 6502 and related CPUs
Blurb: The Blorb packaging specification language
BoxArt: For styling ASCII-art diagrams with boxes and lines
C: The C programming language
C++: The C++ programming language
ConsoleText: For styling command line text in documentation
Delia: For defining test recipes in Intest
Extracts: For code extracts to be written to a configuration file
ILDF: The Inweb Language Definition File format
InC: The Inform-tools extension to the C programming language
Indoc: The markup syntax for the Indoc documentation tool
Inform 6: The C-like interactive fiction language Inform 6
Inform 7: The natural-language based language Inform 7
Inter: The textual form of intermediate Inform code
Inter Pipeline: For pipelines of Inform code-generation stages
Inweb: The markup language for the Inweb literate programming system
None: For programs in languages not yet supported by Inweb
PainterOutput: 
Perl: The scripting language Perl 5
Plain Text: For text files which are not programs
Preform: The internal Inform syntax analysis grammar language
REPL: REPL output in the form expression, colon, value

It's easy to make new language definitions, and contributions of these are welcome.