In these days when business owners are literally bombarded at every turn with advertisements promising that "anybody can build a website in just seconds", you are going to have to make yourself stand out and be very different. You must develop skills you can use to demonstrate to prospective clients that you can deliver more value to them than something they can develop for themselves.
That means you need to transcend the ordinary.
1. Front End Layout Frameworks
There are many layout frameworks you can choose from, so pick a good one and stick with it. The most popular is Twitter Bootstrap because it is well-documented and widely supported by a variety of sources. Using a layout framework will help you speed up development while still keeping all of your work within the sphere of coding. Team this up with a good CMS technology like WordPress and you will have the best of both worlds: rapid site development plus a simple way to manage modular content.
You can learn more about it at the official Bootstrap page. To see a successful implementation of bootstrap, visit the Twitter home page and see Bootstrap functioning in its true home. Not too far off catching up to Bootstrap's popularity, the new kid on the block is Foundation.
2. RGraph
Business clients will love you if you can create real-time charts for them direct from their own databases (but to do this, you'll also have to learn PHP, SQL, and JavaScript). RGraph is a very simple JavaScript plug-in that will work with any website to create beautiful charts of many different kinds. It is very easy to customize and very easy to become a master of it in no time at all.
You can learn more by visiting the RGraph homepage.
3. Layer Styles
While it's not a completely stand-alone technology or one you'll want to mention in your publicity, learning to use the Layer Styles tool properly will give you a way to become super-efficient at creating amazing styles for your CSS divs, so you can rapidly design web pages that have a real wow factor!
It is designed to look like the tool of the same name in PhotoShop, so if you already know how to use PhotoShop for graphic and web design, then you know how to use Layer Styles. You choose the settings, Layer Styles creates the CSS for you. All you need to do then is copy and paste the resulting CSS code into your stylesheet.
4. Raphaël
The HTML5 canvas has really changed the playing field of web development and opened up massive potential for developers who know how to push its limits. Unfortunately most developers haven't got the first clue how to really get the best out of canvas because SVG is very complex and animating is even more so. This is where mastering Raphaël will really help you, because it simplifies the task of SVG coding, and you can use it even without HTML5.
Raphaël was created by SVG expert Dmitry Baranovskiy, and the Raphaël website is now offline after he started working at Adobe Systems. You can still download it from Dmitry's GitHub repository, and you can also see successful implementations of Raphaël at the MCDU Emulator site and Apple iCloud.
5. Google Analytics
Actually any analytics tools are fine, but most business clients will expect Google and unless you can do a really good job of convincing them about an alternative, Google Analytics is probably your best bet. Google provides plenty of useful information, but it's not entirely intuitive. Once you get used to it, you'll be able to add extra value to clients with analytics and SEO advice.