Tally distinct items in a list using the GroupBy operator

String[] items = { "cat", "pear", "apple", "cat", "banana", "pear", "pear", "apple" };

Dictionary<String, int> tallies = items.GroupBy(k => k)
		.ToDictionary(k => k.Key, e => e.Count());

This results in the following dictionary in 'tallies':

{ "cat", 2 }
{ "pear", 3 }
{ "apple", 2 }
{ "banana", 1 }

Join an array of strings

String[] rgs = {"the", "quick", "brown", "fox" };

String s = rgs.Aggregate((av, e) => av + " " + e);
// s == "the quick brown fox", same as:
s = String.Join(" ", rgs);	

Remove all spaces from a string

Note that all of the Linq IEnumerable<T> extensions work directly on a String, processing it as an array of characters.

String s = "four score and seven years ago";
s = new String(s.Where(e => e != ' ').ToArray());
// s == "fourscoreandsevenyearsago"