Digital marketing skills are critical not only to those actually working in the field but also to entrepreneurs. Even if you're past your very early startup days and have a marketing team, the most successful founders have a working knowledge of each aspect of the business and can provide knowledgeable oversight of digital marketing campaigns.

Check out these five top free online courses to build your digital marketing skills:

1. Google Online Marketing Challenge

Over the past 8 years, more than 100,000 people from over 100 countries have participated in Google's Online Marketing Challenge. It's an online course with modules covering introductory digital marketing, search engine marketing, search advertising, display advertising, mobile, social, analytics, and video, making it one of the more comprehensive courses for beginners. However, it's also unique in that Google gives learners a $250 AdWords budget to use over a three-week period, to run an online advertising campaign for a business or not-for-profit. The most successful learners can even win prizes from Google.

If you're looking for a free online digital marketing course with a competitive angle and a real-world experience element, this is a great option.

2. WordStream's PPC University

PPC University is a totally free online learning resource created by my own company, WordStream, to help build your PPC and digital marketing skills. We have access to thousands of accounts spending billions of dollars on PPC advertising, and over the years consistently found that small to medium-size businesses struggled to manage their accounts in a way that was cost effective and earned them the best results. Three streams offer lessons for beginning to advanced users, with additional modules for social advertising.

You can also access a number of webinars and white papers to enhance your digital marketing education, and best of all, everything's mobile-friendly so you can learn on the go, whenever you have time!

3. Social Media Quickstarter Digital Marketing Course

Constant Contact's Social Media Quickstarter emphasizes opportunities to integrate email with social media marketing, to maximize the impact of both channels. In a series of step-by-step lessons, learners can study digital marketing tactics for Facebook, Twitter, LinkedIn, Pinterest, Instagram, Google+, YouTube, blogging, and online listings and reviews sites. The way the courses are structured makes them perfect for beginner-level marketers, with components such as glossaries and how-to guides, combined with practical examples of the lessons at hand in action.

4. Inbound Digital Marketing Course Plus Official Certification

HubSpot Academy offers an incredibly comprehensive digital marketing course that currently has more than 3,000 students participating at Udemy. Over 4.5 hours of instruction in 38 lectures are included in the low, low course price of completely free.

Participants will learn the basics of inbound marketing, SEO, landing pages, blogging, conversion optimization, lead nurturing, and email marketing, as well as how they all work together for a cohesive and more effective inbound marketing strategy. After the certification exam, graduates receive a badge for their résumé or LinkedIn profile to show they've expanded their marketing skills with HubSpot Academy.

5. Alison Free Diploma in E-Business

Alison is a massive online learning community of more than six million registered users, with standards-based and certified courses offered absolutely free of charge. Developed by free online courses pioneer Mike Feerick, Alison was founded in 2007 and was the first MOOC (massive online open course) provider.

Today, you can take the free digital marketing course Diploma in E-Business to build skills in search optimization, Google Analytics and AdWords, campaign tracking and integration, revenue metrics analysis, digital measurement, and more.

And, of course, if you want to make yourself truly invaluable to any organization (including your own startup), you need to learn how to code!