Q:

How to make even small website worth of UML modelling?

I've been thinking to create a small UML model as I wish to recreate the website with online games, videos, iRadios, etc. However, the website seems to me to be too simple to waste UML modelling on it. 
First, I thought of activity diagram to specify the "life" of user after login or after registration, but both these activities seem to be so simple to me, that I'm missing a point of using the UML do depict the process of such simple tasks.
Please, give me your ideas how to use UML modelling to help a simple project efficiently.

A:

There are several good discussion about the uses of UML on SO for example here.
I would apply UML to designing a small Web Site in two ways:
1). Document the user's experience using a state diagram. Each state represents a situation where the user can take an action (a "screen" in simple Web Apps) . The state transtions represent movements to places where users can do something else (press checkout, now we're on payment screen),
The point of this is to have a very clear idea of the overall navigation of your app. You can now associate classes with each state, the classes representing the data on the screen.
Is such a diagram worthwhile? All depends how "obvious" your app is and how big your team is. Doing this up-front design is pretty essential for any team size > 1. For all but the most trivial of apps it's also worth doing even for a one-person team. Thinking before coding is usually good. If you have UML tools available they are probably easier to use than doing the same thing in Visio.
2). OO design of the tricky bits of business logic. UML class diagrams. Again we are doing up frant design rather than hacking code. If you have no tricky bits then you may not need this.
I see sequence diagrams as useful mainly for communicating tricky aspects of the design amongst team members. Rarely useful for one person projects.

