Good example for a custom service application

Check out chapter 9 of the book Microsoft SharePoint 2010 Web Applications: The Complete Reference, there is a really good example how to create a custom service application. The described scenario creates a translation service and a webpart that's using it.

But be aware of a little error on page 199, change this line

public string AlternativeLanguage { get; set; }

to 
[Persisted] private string _alternativeLanguage;

public string AlternativeLanguage
{
   get { return _alternativeLanguage; }
   set { _alternativeLanguage = value; }
}
or otherwise no change of the Alternative Language would be stored.