Unless you have an older plan, you likely just have access to Mobile Web, rather than free use of it. It's free in the sense that you have it and can use it, but you still get charged for the data you use. I believe a now-grandfathered plan had unlimited Mobile Web, but I'm not sure whether you've got that or not.
What you can do with data depends more on what phone you've got. If you have a phone with an HTML web browser, you can use that freely without incurring data charges. It's used for downloading apps, music, and such, too. I could be wrong about this, but I believe VZ Navigator is a separate service, so I don't think it's covered. It should offer unlimited use of Mobile Web, downloads from Verizon's store, and...well, not much else unless you have a smartphone. Data is something generally more geared towards smartphones. If you don't expect to use Mobile Web much, and you don't have an HTML web browser on your phone, it's probably not worth paying for unlimited data.
Thanks. I have an alias2. Does that have the browser?
The Alias 2 has a simple WAP browser. It's not a full-featured browser like what you'd find on a smartphone or select others. You should be able to view HTML pages, but it's most likely something that would be an unpleasant experience due to the screen size, and improper formatting that would likely pop up. You'd have to try it and see. It's basically just Mobile Web.
So you probably won't be using your phone to browse the web, unless you somehow find it tolerable on that phone. I've got an iPod touch, and I personally couldn't imagine browsing the web on any lesser mobile device. It depends on the person, though.
The $29.99 unlimited data plan gives you unlimited access to the internet. If you have a smartphone, such as the Droid, you can basically do almost anything you can do on the internet with a computer. That includes email, surfing, uploading and downloading, etc. Some of the phones can also be tethered to your laptop to provide internet access on your laptop, but this plan runs $60 per month. If you have no need for internet access, except email for instance, you could cut back to the $10 data plan. If you have no need at all for internet access, you can delete the data plan altogether, but you would have to get a simple phone that does not require a data plan.