there are a few listed on try matrix now and the bots page: https://matrix.org/docs/projects/bots/

maubot's readme has a mostly up to date list of existing maubot plugins: https://github.com/maubot/maubot
go-neb's readme lists all the modules it has: https://github.com/matrix-org/go-neb

if your definition of bot includes bridges, there's a fairly good list of bridges on matrix.org: https://matrix.org/bridges/

In reply to @tulir:maunium.net
that's also possible if you self-host it and set the option to do that

{
   text: 'title of XKCD',
  attachments: [
    'image post id',
    'alt text id'
  ]
}

{
   text: 'title of XKCD',
  attachments: [
    'image post id',
    'alt text id'
}

{
   text: 'title of XKCD',
  attachments: [
    'image post id',
    'alt text id'
  ]
}