Dictionary API (lexical)

I'm with Barry Fandango on this one, but you can do it without LINQ. .NET 2.0 has some nice filtering methods on the List(T) type. The one I suggest is

List(T).FindAll(Predicate(T)) : List(T)

This method will put every element in the list through the predicate method and return the list of words that return 'true'. So, load your words as suggested from an open source dictionary into a List(String). To find all words of length 5...

List(String) words = LoadFromDictionary();
List(String) fiveLetterWords = words.FindAll(delegate(String word)
    {
        return word.Length == 5;
    });

Or for all words starting with 'abc'...

List(String) words = LoadFromDictionary();
List(String) abcWords = words.FindAll(delegate(String word)
    {
        return word.StartsWith('abc');
    });