Sebastian Seilund Sebastian Seilund

My work

These days my primary work is my consulting business. Contact me if you have a project in mind. I would love to hear about it!


I help companies build quality software. I specialize in web applications. Any kind of webbased software that services users, munges data, processes payments, you name it. I do both conceptual work, back-end and front-end. I can also help with design and UX work through people I work with.

I work both on completely new projects building software from the ground up, but can also help optimize aspects of your existing systems.

I'm working on getting some kind of portfolio up on this site. One cool site I've been working on recently is If you would like to know more, please contact me.


Billy is an online accounting software that I founded in 2010. We make it simple for our users to manage their accounting. Users can send invoices, manage bills, reconcile their bank and much more. Our vision is to automate as much as possible while providing valuable business insights. We have a very attractive free plan with no usage limits and a premium version that offers extra features and automation.

Our Danish beta went online in Denmark in April 2011 and in August 2011 we started accepting customers. In 2016 we're focusing on the US market and the rest of the world.

Billy's front-end is a large Ember.js app, which we started working on in the summer of 2012 (when Ember.js was still on v0.9.8). This has brought me an extensive experience with Ember.js and large-scale webapp development in general.

The back-end has over the past few years evolved into a microservice-oriented architecture. We currently run about 40 services primarily written in Node.js, though we've already put a few Elixir services into production and hope to continue with more Elixir in the future. Several new parts of our system use event sourcing and CQRS. All our services run as Docker containers in our AWS cluster managed by Apache Mesos and Marathon. We have a single public REST API that we use for both our own webapp and external applications.

Today I function as an external advisor to the Billy development team.


I taught myself programming somewhere around year 2000 and have been hooked ever since.

Before founding Billy I spent most of my time doing consulting for other companies and occasionally some of my own projects, too.

Here are some of the fun projects I've worked on in the past:

  • An ad space auction site.
  • A system that manages bookings for a rental company.
  • An online portal where parents of school kids can order lunch for their children.
  • A system that analyzes live sports data and provides real-time feedback to users.
  • Countless webshops.
  • An ERP system for a company that sends weekly newsletters to paying subscribers.
  • A photo-sharing service.