There a different ways to help new visitors understand your service and how to use it. This is a collection of such onboarding concepts. No analysis, just some screenshots.
To Do Lists
Give the user a list with things they can do with your service.
dropbox
Dropbox has the incentive of 250MB of free space for everyone who fullfills their list of todos. The actions make sure that the user understands the service and what she can do with it.
angel.co
Angellist is standardized. All startups need to input the same types of information and so it is easy to give them a todo list. Every time one visits their page they see the todo list and have the urge to finish it. This doesn’t mean the user understands angellist but at least they got a nice profile which makes the service more useful for others. And angellist is that specific that nearly everyone who signs up knows quite exactly how to use it.
Live Demonstration
Show them how to use the service by letting them use it.
buffer
Buffer does a great job at that. After the sign up it presents you an example tweet that you can shedule for later. Buffer has the benefit that this is all it does, sheduling tweets (what it does quite well), and the action doesn’t cost the user anything. 
Video
This may be quite boring but some users love videos that show how to use the site. I don’t like them that much because they don’t give much
dropbox
Again.
Tutorials
This isn’t really onboarding. More complex sites often have extensive tutorials how to use the service. Often accompanied by best practices and a forum for user to write with others and help themselves if something doesn’t work as expected.
Product Tour
Take the user at the hand and show her one thing after another. If you have mercy you don’t use more than six steps for this. If you are smart you let the user abort the tour and restart it whenever they want. If you are a smartass you give them a goodie for taking the tour.
plurk
http://whitneyhess.com/blog/2008/06/20/plurk-you/
yahoo shine
http://whitneyhess.com/blog/2008/03/31/yahoos-shine/
dropbox
You can also make a little book that shows the user what your service can do for them.
Visual pointers
This is the nicer version of product tours. Instead of taking the user by the hand you just point at the most important elements on the site and let the user try them themselves.
Google Maps
http://www.quora.com/What-websites-have-the-best-new-user-onboarding-flows
Force
This is a bit evil. You force the user to make certain actions and don’t let them do anything else.
Twitter forces their user to follow a certain number of people before they let them tweet something. It is quite annoying if you set up a new account but it seems to help them get the retention rate up. You can skip this by reloading the page which most new users won’t do. There is also a small skip this step link. I don’t have the exact number by hand but they said that people stay become regular users most often when they follow 10 people and get 5 follower or something like that.
Sources:
http://whitneyhess.com/blog/2009/10/06/onboarding-a-sidebar-in-designing-social-interfaces/
http://www.knewton.com/blog/knewton/inside-knewton/2012/02/06/ux-corner-5-great-onboarding-examples/