Converting a string to lower-case (without built-in to-lower functions!)

Python 2.7 - 30 (with terrible and unapologetic rule abuse)

raw_input().upper().swapcase()

As an anonymous edit pointed out, you can do it in 27 26 in Python 3:

input().upper().swapcase()

I'm flagrantly abusing the rules here, but...

Important: you are NOT allowed to use a built-in function that converts the string (or just one character) to lowercase (such as ToLower() in .NET, strtolower() in PHP , ...)! You're allowed to use all other built-in functions, however.

This takes the strings and coverts it to upper case. Then in a very unrelated method call, it reverses the case of the string - so that any lower case letters become upper case letters... and swaps any upper case letters to lower case letters.

Perl - 11 10 characters.

y/A-Z/a-z/

y/// is same as tr///!

In action:

% perl -pe 'y/A-Z/a-z/' <<< 'Hello @ WORLD !'
hello @ world !

Shell - 10

Translation of @Gowtham's Perl solution using /bin/tr.

tr A-Z a-z

Sample run:

% tr A-Z a-z <<<'Hello WORLD! @'
hello world! @