Prime Sieve. No arguments about whether 1 is prime... :p

sieve.scala

import java.util.Calendar;

def sieve(primes:List[Int], possible:List[Int]):List[Int] =
    possible match  {
        case Nil => primes
        case firstPrime :: xs => { 
            sieve(firstPrime :: primes , xs.filter( _ % firstPrime != 0) )
        }
    }

val start = Calendar.getInstance().getTimeInMillis();
val res = sieve(Nil,(2 to 100000).toList)
val end = Calendar.getInstance().getTimeInMillis();
println( "Found %d primes in %f seconds.".format( 
  res.size ,(end - start) / 1000.0 ) 
);

// scala sieve.scala 
// Found 9592 primes in 2.777000 seconds.