RemoveAtFast

public static class ListExtender
{
    /// <summary>
    /// Fast version of the RemoveAt function. Overwrites the element at the specified index
    /// with the last element in the list, then removes the last element, thus lowering the 
    /// inherent O(n) cost to O(1). Intended to be used on *unordered* lists only.
    /// </summary>
    /// <param name="_list">List.</param>
    /// <param name="_index">Index of the element to be removed.</param>
    public static void RemoveAtFast<T>( this IList<T> _list, int _index )
    {
        _list[ _index ] = _list[ _list.Count - 1 ];
        _list.RemoveAt( _list.Count - 1 );
    }
}

using ListExtender;

void Test()
{
    List<int> intList = new List<int>( 10000 );

    // Populate list
    for( int n = 0; n < 10000; ++n )
    {
        intList.Add( n );
    }

    // Remove all elements. Note that not a single memory 
    // xfer will be performed since internally we're 
    // always removing the last element regardless the 
    // index we passed in.
    for( int n = 0; n < 10000; ++n )
    {
        intList.RemoveAtFast( 0 );
    }
}