I mean "hackable" in the sense that one can decide to experience it in ways that were not exactly what the author decided it would be. In short, the Web is not TV. It's not PDF either. Nor Flash.
A couple of months ago, we had this discussion during the Mozcamp in Utrecht. It's hard to summarize all of this in a blog post, but I'm going to give it a try.
What's cool for the (Open) Web is that one can tweak/change/hack most of the pieces of the stack. Of course, some of the pieces are out of reach (the DNS servers, the Web server, most of the network) and it's good. But for a lot of the pieces, the users has – if he wants – the ability to change the pieces in order to fit his needs. This sounds a little complex? Let's use examples:
- Changing the look of the document via CSS : you can use User Stylesheets (even better and easier with Stylish)
- Changing the content via user scripts, implemented via Bookmarklets, GreaseMonkey or Jetpack.
- Change the look of the browser using Themes for your browser or Personas
- Change the way you interact with the browser, with add-ons such as Ubiquity, which completely redefines how we interact with the Web browser and the Web itself.
The beauty of all this is that the people who have invented this did not have to ask permission to innovate. The way the Web was invented, with standardized layers, enable these kinds of things, and it's good.
This "hackability" (or generativity) is one of the key things I love about the Web. Now the issue is that this key ability does not have an actual name. Mark Surman has a good post on this topic. Should we call this essential "characteristic" about the Web "Generative", "remix", "opportunity", "hackable", "permissive"? Go and read Mark's post and comment here or there!
 The Web was invented 20 years ago, and bookmarklets became somewhat popular in 2002, GreaseMonkey was popular in 2005, Ubiquity Alpha was released in 2008 and Jetpack was announced a couple of weeks ago! No one knows what's going to be invented thanks to the generative nature of the Web...