Freelancing

Art of Emails is a rich source of email templates and tips for freelancers. If you haven't had much luck sending applications on Upwork-like platforms, check their take on proactively reaching out to clients.
They also have several email templates and examples on solving typical freelancer challenges: getting referrals, finding new clients, deciding on whether to do unpaid work.
Explore their website to find many more templates that you can download, on topics like link building, partnering with influencers, fundraising, converting users, getting paid work during the COVID-19 pandemic, and many more.

If you want to work for the non-profit sector but at the same time be your own boss, in command of your schedules and projects you work in, Work For Impact is your answer.
This platform aggregates opportunities from socially and environmentally-conscious organizations that aim to positively impact the world.
Their blog has interesting articles with perspectives on the cross-over of the nonprofit and the freelancing worlds.

Toptal prides itself in having in their network the top 3% of freelance talent, from designers to developers, product-managers finance experts, and project managers. Companies and businesses can approach Toptal to request a top-notch team, and Toptal selects the right talent from their pool of freelancers. If you are a freelancer in one of these five areas, you can apply to join this pool. You will then have to go through a series of rigorous screening tests about skills, languages, and personality and complete a test project. While it seems a bit daunting, if you are successful, you are rewarded with great work opportunities.

In this website, companies who need talent post jobs. Freelancers can then bid for the gig and, if selected, can then complete it and receive their payment securely. The website charges a 10% (or USD 5, whichever is greater) fee from the freelancer.

Upwork is a very popular website to search for work as a freelancer, with job opportunities spreading across multiple areas and both short and long term projects. When a hirer posts a job, the freelancer can apply using "connects" (each freelancer gets a fixed number to use for applications and these renew every month). The platform includes an online chat, making it easy for hirers and freelancers to communicate and negotiate. Secure payments are sent to the freelancer either per mile of accomplished work (agreed with the hirer) or when the project is complete. Upwork charges the freelancer 20% for the first USD 500 billed from a client, 10% up to USD 10,000, and 20% when it exceeds 10,000 (note that for each new client, Upwork will always charge 20% for the first USD 500, etc.).

Fiverr is a big marketplace for freelancers to sell their services. Jobs are divided into the following categories: graphics and design, digital marketing, writing and translation, video and animation, audio and music, tech, business, and lifestyle. It's a great place whether you have a service you can sell or are looking to purchase a gig. Prices tend to be low but can vary between $5 and $10,000. While Fiverr is a great place to sell your services if you are, for instance, an SEO expert or a logo designer, some other peculiar gigs can also be found there, such as online game coaching, celebrity impersonators and plenty of others worth to explore in the "Lifestyle" section.