One of the big differences between Ruby/Rails and the .Net world is the fact that most of the available plugins are open-source and integrate at the code level. There is an incredible array of plugins for Rails, and it is very straight forward to write your own. Due to the nature of Ruby you can hook into any just about any part of the language and framework, giving you impressive extensibility.
I am not sure how Web Controls work, but it sounds like they are a "black-box" that provides an end-to-end solution for both UI and data-level operations ... ?
Many of the Rails plugins do provide both UI and data aspects. An example would be "restful_authentication" which provides you with both some basic forms for login and user registration as well as an authentication module and a Active-Record model. Again, this operates at a code-level, so will actually push the relevant code into your codebase when you install and "generate" the authentication modules.
As for "widgets", there is no equivalent in Rails, per-se, but there are a number of JavaScript libraries that provide similar functionality. I use and recommend jQuery UI, myself.