Windows Phone already make it easy to pin a website to the Start Screen (choose "pin to start" from the main menu in Internet Explorer). However, the Live Tile image used for such bookmarks is a thumbnail of the current web page view, and the name of the Live Tile is drawn from the page's title. In some cases this looks absolutely fine, but in others, especially for complex web pages, it can look rather messy.
What Appify does is to allow greater control over the Live Tile web bookmark creation process. The first step is to enter a website address. The specified website is shown in a browser control within the Appify app, so you can confirm it is the right site.
The second step is to provide a name for the Live Tile (it's worthwhile keeping this relatively short, so that it fits on the finished Live Tile), the third step is to provide an image (drawn from those stored on your phone), and the final step is to pin the Live Tile to the Start screen.
One limitation that's worth noting is that the Live Tile will open the bookmark in its own browser control, rather than Internet Explorer. That's due to a limit of the Windows Phone APIs, rather than the way the app is written.
If you don't like the way Appify does things, there are a number of similar apps in the Marketplace including Bookmark Tile Creator and WizTiles. You can also use a JavaScript based trick to use a website's favourite icon for a similar purpose.
Marketplace description:
Your favorite site does not have WP7 app? Not a problem any more! With this extra easy-to-use app, you can transform ANY websites into pinned apps in seconds. Just enter the site's address, pick an image for the tile, choose a snappy name, and there you go!
Appify can be downloaded from the Windows Phone Marketplace for free. However, the developer says that only he first 10,000 downloads will be free, after which Appify will become a paid app.