What's your favorite LINQ to Objects operator which is not built-in? [closed]

Answer 1

IndexOf

/// 

/// Returns the index of the first element in this 
/// satisfying the specified . If no such elements
/// are found, returns -1.
/// 
public static int IndexOf(this IEnumerable source, Func condition)
{
    if (source == null)
        throw new ArgumentNullException("source");
    if (condition == null)
        throw new ArgumentNullException("condition");
    int index = 0;
    foreach (var v in source)
    {
        if (condition(v))
            return index;
        index++;
    }
    return -1;
}