A key tool for any budding web developer is API knowledge. It's good to familiarise yourself with a variety of application interfaces. I started out using the Twitter REST API with PHP and CURL when twitter first launched the API to basically get a users tweets, pretty simple now I look back, but at the time it was the first time i'd done such a thing. As the development of social media continues this will increasingly open up opportunities to develop more and more applications that interact with one another and and make our daily chores more eventful.
Companies are Increasingly seeing the massive potential that there is in the mobile platform. Applications that are approved and make it into the Apple app store have the opportunity to be downloaded thousands of times and can potentially make a lot of money. Having a specific skill to be able to develop for this market can be very lucrative, especially in the current economic climate when companies are turning to social media to plug holes where they're not seeing return on investment from other advertising sectors.
A handy skill to have is a general knowledge of how to use content management systems. For example building themes and plugins for WordPress, or modules and components for Joomla and Mambo. As a developer its important to have an idea of the structure of a variety of CMS's. If you have a wordpress blog/site, that's an easy way to familiarise yourself with the platform. Remember it's always easier to learn something by immersing yourself in the technology.
Businesses are always going to need payment solutions online. Having the skills to take a site and integrate paypal or google checkout will allow you to go far. We're starting to see many 3rd party software companies setting up and selling software at more affordable prices, take Panic for example. They are a relatively small company that develop a selection of web development software for the mac. Many new startups are settling for a product based revenue stream rather than making money purely through ads, revenue share, or partnership deals. This opens up the demand for skilled developers to be able to build payment solutions.