In the last few months, since I asked the question, quite a lot of free options appeared.

I'll try to list them in this community wiki:

AdminLTE Dasboard and Control Panel Template

• site: http://almsaeedstudio.com/
• live: http://almsaeedstudio.com/preview
• repo: https://github.com/almasaeed2010/AdminLTE
• repo activity: HEALTHY (Checked 24-oct-2016)
  • Last commit (not counting merges) on 19-oct-2016 = 5 days ago (from date of check).
  • Number of forks: 4.515 (at date of check).

    * license: MIT

Robust Lite : Free bootstrap admin template

• site: https://pixinvent.com/products/robust-lite-free-bootstrap-admin-template/
• live: https://pixinvent.com/free-bootstrap-template/robust-lite/html/ltr/index.html
• repo: https://github.com/pixinvent/robust-free-bootstrap-admin-template
• repo activity: HEALTHY (Checked 16-june-2017)
  • Last commit - 2 month ago (from date of check).

    * license: MIT

Metis

• site: http://onokumus.com/?p=63
• live: http://demo.onokumus.com/metis/
• repo: https://github.com/onokumus/Bootstrap-Admin-Template
• repo activity: HEALTHY (Checked 24-oct-2016)
  • Last commit (not counting merges) on 16-sep-2016 = 1 month ago (from date of check).
  • Number of forks: 1.053 (at date of check).
• license: MIT

DevOOPS

• site: http://devoops.me/
• live: http://devoops.me/themes/devoops/
• repo: https://github.com/devoopsme/devoops
• repo activity: OLD (Checked 24-oct-2016)
  • Last commit (not counting merges) on 20-Nov-2015 = 11 months ago (from date of check).
  • Number of forks: 349 (at date of check).
• license: GPL v3 or MIT

SB Admin

• site: https://startbootstrap.com/template-overviews/sb-admin/
• live: https://blackrockdigital.github.io/startbootstrap-sb-admin/
• repo: https://github.com/BlackrockDigital/startbootstrap-sb-admin
• license: MIT

SB Admin 2

• site: http://startbootstrap.com/sb-admin-v2
• live: http://startbootstrap.com/templates/sb-admin-v2/
• repo: https://github.com/IronSummitMedia/startbootstrap/tree/master/templates/sb-admin-v2
• repo activity: DEAD
  • This link is 404 Page Not Found on 24-oct-2016.
• license: apache 2

Light Way Admin

• site: http://www.prepbootstrap.com/bootstrap-theme/lightway-admin
• live: http://www.prepbootstrap.com/bootstrap-theme/lightway-admin/preview/index.html
• repo: ?
• license: MIT

Deep Blue Admin

• site: http://www.prepbootstrap.com/bootstrap-theme/deepblue-admin
• live: http://www.prepbootstrap.com/bootstrap-theme/deepblue-admin/preview/index.html
• repo: ?
• license: MIT

Dark Admin

• site: http://www.prepbootstrap.com/bootstrap-theme/dark-admin
• live: http://www.prepbootstrap.com/bootstrap-theme/dark-admin/preview/index.html
• repo: ?
• license: MIT

And here's an article that lists ten more free admin bootstrap templates:

http://dizyne.net/free-bootstrap-admin-templates/

Bootstrap is less of a javascript library and more of a CSS framework with some additional javascript widgets. There is nothing preventing you from using jQuery or some other javascript library of your choice (indeed, it may be required to get the functionality you need.)

The use case for Bootstrap is to get the product out the door without spending time worrying about how each control should look. It defines some standard styles for common elements/widgets and styles them to look good. It uses best practices and common sense to get a uniform look for everything.

There is absolutely nothing that is ASP.NET MVC specific about Bootstrap. It is pure front-end goodness, so it doesn't care in the slightest on how you render the HTML to start with. The entire purpose of Bootstrap is to bootstrap (get it?) your user interface.