As it has been with most Google products, after immense testing (read teasing) the general public with an awesome service, Google has finally opened up Google Voice for everyone without having to wait for an invite.
Yes, the Google Voice wait is finally over. You can go ahead and sign up for a new account right away at https://www.google.com/voice/ and you will instantly get a Google Voice number. However, this will only work for US users right now.
Before you start out with Google Voice, you have to understand that it is not really straightforward to understand and use the service, so you might want to head over to the features page to get an understanding of how the service works and what you can do with it.
For a quicker overview, check out the video below which tries to explain in simple terms what Google Voice is all about.