A "run time RI for ruby methods", this gem allows you to query for information about objects' methods within irb or a ruby-debug prompt.  It reveals everything known about the method.  This includes source, ri, arity, rdoc comments (on 1.9), etc. where available 

For me it has proved quite useful, and I wouldn't do a ruby-debug session without it--try it--you might really like it.

Examples:
>> class A; 
    def go(a); end; 
   end
>> A.desc_method :go
#<UnboundMethod: A#go>      arity: 1
ri for A#go
Nothing known about A
(end ri)
def go(a)
  # do nothing
end
Parameters: go(a)
>> File.desc_method :delete
ri for File.delete
----------------------------------------------------------- File::delete
     File.delete(file_name, ...)  => integer
     File.unlink(file_name, ...)  => integer

     From Ruby 1.9.1
------------------------------------------------------------------------
     Deletes the named files, returning the number of names passed as
     arguments. Raises an exception on any error. See also +Dir::rmdir+.

(end ri)
#<Method: File.delete>     arity: -1
appears to be a c method
#parameters signature: delete( [[:rest]] )


Or (my favorite) using it in a debug session:
(rdb:1) l=
...
=> 74       assert(assigns['order'].order_line_items.map(:unit_price).min >= -5)
...
(rdb:1) desc_method :assert
#<Method: StoreControllerTest(Test::Unit::Assertions)#assert>      arity: -2
ri for Test::Unit::Assertions#assert
------------------------------------------ Test::Unit::Assertions#assert
     assert(boolean, message=nil)

     From gem test-unit-2.0.1
------------------------------------------------------------------------
     Asserts that +boolean+ is not false or nil.

     Example:

       assert [1, 2].include?(5)

(end ri)
def assert(boolean, message = nil)
  _wrap_assertion do
    assert_block("assert should not be called with a block.") do
      (not block_given?)
    end
    assert_block(build_message(message, "<?> is not true.", boolean)) { boolean }
  end
end
Parameters: assert(boolean, message = nil)
(rdb:1)

========= How to Install=====
$ gem install rogerdpack-desc_method 
or
$ gem install rogerdpack-desc_method --source http://gems.github.com 
if you don't have gems.github.com as a gem source.

Usage:
>> require 'desc_method'
>> Class.desc_method :method_name # class or instance method name
...
>> some_object.desc_method :method_name
...

Other goodies also included:
Class#desc_class

Examples:
>> Object.desc_class
# outputs descriptive info about that class--RI, method list, etc.
>> Object.desc_class :verbose => true
# outputs RI, method lists including inherited methods, ancestors, all constants etc.

Kernel#methods 
This has been monkey patched to output a "separator" between its inherited and non inherited methods--i.e.
>> instance.methods
=> [:first, :second, :after_this_are_inherited>>>>>, :some_inherited_method, :another_inherited_method] # adds in that separator

Though I'll probably remove that in a future release since it turns out you can just run Kernel#methods(false) for about as much usefulness.

=== Thanks ===
This gem wraps for convenience the functionality of Method#source_location, ruby2ruby, et al, and also contains some code inspiration from manvenu, SourceRef (MBARI), and Python's Method#desc.  It also wouldn't be useful without irb and the ruby-debug folks. Thanks!

Comments/suggestions welcome rogerdpack on gmail or github