On the main navigation bar right after the Tasks tab, you'll find the Projects tab. Here you have a list of all the projects in your account. You can order them alphabetically, by create date, update date or status (open or closed). Open projects are active projects, people are adding notes and tasks to them. A closed project means nobody can attach notes or tasks to it.
There are two ways to add notes to projects.
First, from the project page. Here you can optionally select a contact involved in this project (click the Show Options link to view the contacts list). If a contact is selected the note will be attached to both the selected contact and the project. Otherwise, it will just be attached to the project.
Second, on a person or company page. Here, in the add note options panel you have a list of your open projects. You can select one of these projects when you add a note to attach the note to both contact and project.
You can also add tasks about projects by adding a task on the project's page.
I hope you'll find this new feature as great and as useful as I do.