I recently discovered   = in ruby. Its main use is for initialising a variable if it does not already exists, this is great for extending templates with new functions and maintaining backwards compatibility. 
irb(main):001:0> x 
NameError: undefined local variable or method `x' for main:Object
from (irb):1
irb(main):002:0> x ||= 0
=> 0
irb(main):003:0> x = 10
=> 10
irb(main):004:0> x ||= 0
=> 10

I previously used :

if @x.nil? 
    @x = 10   
end

which is 3 times as many lines of code and does not work with all variable types infact .nil? is only valid for global ($) and instance (@) variables.

irb(main):001:0> $x.nil?
=> true
irb(main):002:0> @x.nil?
=> true
irb(main):003:0> @@x.nil?
NameError: uninitialized class variable @@x in Object
   from (irb):3
irb(main):004:0> x.nil?
NameError: undefined local variable or method `x' for main:Object
   from (irb):4