In Rails? Yes. In gems? Uncommonly. In pet projects/experiments? Probably a lot. Usually people are just adding convenience methods to a class rather than changing behavior of existing methods.
For example, I want to be able to quickly take the average of an array, so can I could define Array#average that would handle that rather than passing an Array to an average method.
I think everyone uses something different for different reasons. I prefer sublime at home and a combination of sublime/rubymine at work (since I get the license for free and it's REALLY useful for debugging very large applications).