I'll have to write something about Blueprint. It's a CSS framework which gives you a complete grid layout for free. I'm using it for a site I'm working on right now and it's looking great. The usefulness of such a framework is pretty obvious, but I'll tell you why it's a big deal anyway. You don't have to invent the wheel again, thus saving a lot of development time. It's flexible, so if you decide to change the layout in a later stage your whole design doesn't fall apart.

Finally, and in my opinion the most important point, it's tested. It supports most browsers (even IE 5 with some special markup). Also it's one of the more tricky parts of web development. If that is not a time saver, I don't know what is...