These are apps, web services, and plugins that I use, love, and highly recommend to fellow authors. Some of the links below are affiliate links, but I only signed up for these programs after using the products extensively myself.
I draft every novel and nonfiction book in Scrivener. It's a wonderful program with a ton of features. I admit I only use a few, but they are crucial to me and my process at this point. Plus, for those who don't want or aren't able to get Vellum, Scrivener is a good alternative for formatting.
Evernote is a robust note-taking app which allows you to organize text, images, links, and more. The best part about it is that you can access it from anywhere since it is cloud-based. I use it to organize notes, research, ideas, and more, as well as to store portions of my WIPs when I want to be able to draft on the go.
Evernote has both free and premium versions. I have used both and love both. The free version is definitely adequate, but the premium version includes features like a higher image allowance and a higher number of registered devices.
Vellum is book formatting software which allows you to quickly and easily create beautiful eBook and print book files in a variety of design styles. Plus, you can drag sections from one file to another with a couple of clicks, which makes updating your author bio, book links, etc. incredibly easy.
Vellum is Mac-only with no Windows version in the works. But personally, I bought a refurbished Mac just to use Vellum, and I don't regret a single thing. Vellum is invaluable software, and anyway, I'm a Mac convert now.
PayHip allows you to sell your own memberships and digital products, which of course includes eBooks. It's different than selling your book through a store like Amazon because PayHip only takes a small cut of your profits (5% on the free plan but as little as 0 on a paid plan), and you get those profits immediately. Plus, you can create coupon codes for your readers. (And there are a few other cool features, too.)
Like Draft2Digital, PublishDrive is an aggregator which you can use to distribute your book to multiple stores. While I use Draft2Digital to distribute my books to most non-Amazon stores, D2D lacks the ability to get my books into Google Play. For this, I use PublishDrive.
For Your Author Platform
MailerLite is the email service provider I use it to manage my mailing list. Previously, I used another provider, but I switched to MailerLite for its affordable pricing and clean, easy-to-understand interface. It has all the tools you need to collect signups and create well-designed email newsletters and landing pages.
Elementor is a WordPress plugin and page builder. It builds upon your existing theme but allows you to create completely custom pages using drag-and-drop elements. You can also create templates and global elements for use across your site. My favorite thing about it is that you can create multiple headers and footers to go with different sections of your site.
Elementor has a robust Free version with additional drag-and-drop elements included in the Pro version. The Pro version is well worth it for the forms functionality alone.