http://rubysource.com/ruby-golf/

golf.rb

# Byte counts don't include newlines, def... and end

# Hole 1: 61 bytes
def fizzbuzz(n)
  "FizzBuzz#{n}"[(n%3<1?0:(n%5<1?4:8))..(n%5<1?7:(n%3<1?3:-1))]
end

# Hole 2: 72 bytes
def caesar(s,n)
  s.tr "a-zA-Z",(h="#{(a=(?a..?z).to_a*'')[n%=26]}-za-#{a[n+1]}")+h.upcase
end

# Hole 3: ... nah

# Hole 4: 20 bytes
def count(s,u)
  s.scan(/#{u}/i).size
end
# If you want to allow special characters, you have to do
#   s.scan(/#{Regexp.quote u}/i).size
# but this works for most cases