Rails allows you to choose your own testing framework [1][2][3] and plugins[4][5][6], templating engine[7][8][9], and ORM[10]. It's my understanding that before Rails 3, swapping out many default Rails was a bit hacky, but it's pretty standard now.

[1]: https://github.com/rspec/rspec-rails [2]: https://github.com/cucumber/cucumber-rails [3]: https://github.com/thoughtbot/shoulda [4]: https://github.com/thoughtbot/factory_girl [5]: https://github.com/nulldb/nulldb [6]: https://github.com/notahat/machinist [7]: https://github.com/haml/haml [8]: https://github.com/josh/mustache-rails [9]: https://github.com/stonean/slim [10]: https://github.com/brasten/sequel-rails [11]: https://github.com/datamapper/dm-rails [12]: http://mongomapper.com/documentation/getting-started/rails.h...