Trello, the web-based project and task-management system, announced on Thursday that it now has more than 5 million users.

Functioning much like a visual to-do list, Trello shows what needs to be done, and what is happening at various stages of a project or assignment. Think of it as a 21st-century interpretation of a Post-it board.

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Trello originally launched in 2011 as a product under Fog Creek Software. The aim was to build a tool that could appeal to developers and non-developers alike. Ultimately, it was a success, with users and businesses using Trello for everything from project management and product roadmaps to shopping lists and wedding planning. At Mashable, we use Trello to manage our editorial workflow in the newsroom.

The Trello product itself is even developed using Trello.

Trello is a web app, but it also has native mobile apps for iOS, Android, Windows 8 and Kindle Fire. As a user, the mobile apps are one of my favorite features because it means I can edit cards or tasks, get notified of changes to a card and receive alerts when I'm mentioned on another card in real-time.

In July, Trello raised a $10.3 million Series A round, and was formally spun out of Fog Creek into its own company, Trello, Inc. In a blog post about the spin-off, Fog Creek CEO Joel Spolsky said having funding and its own company will enable Trello to grow faster and develop more features.

Image: Trello

The core Trello product is available for free, but it also offers Trello Gold as an option for users who want to take advantage of bigger file uploads, stickers, custom backgrounds and custom emoji support. There is also a Business Class offering for teams that want more granular control over logins, bulk-data exports and administrative-level controls.

Still, most of the 5 million Trello users use the free version, and the company hasn't made a big push to convert users into paying customers. Instead, its goal from the start has been to build organically. Almost all of Trello's user growth is organic, coming from word-of-mouth, rather than outbound or inbound marketing campaigns.

In fact, it's only now — with its own separate company, new CEO Michael Pryor and a Series A round — that Trello seems ready to spread the word.

Trello has let its members earn free Trello Gold time by spreading the word about the company on social media. Just like earning free space on Dropbox, users get more Trello Gold time for each new member that signs up. To mark its 5-million-member celebration, users can earn a month of free Trello Gold by sharing the announcement on Facebook or Twitter.