Shortest Floor Function

PARI/GP (10)

In gp (and some other languages) x%1 gives the decimal part of x:

f(x)=x-x%1

NOTE: For negative x, x%1 returns (1 - abs(decimal part of x)), so the above works both for positive and negative numbers.

JavaScript, 50 34 33 32 characters

function f(n){return~~n-(~~n>n)}

Works the same way as the PHP one I submitted.

DC (15 bytes)

Makes use of a nifty little trick that occurs during division in DC. Add a 'p' to then end to get output (it performs the floor correctly anyway), I assume that stuff is not already on the stack, and that input is in stdin.

[1-]sazk?z/d0>a

EG: echo 0 2.6 - | dc -e '[1-]sazk?z/d0>ap'