JobberBase: open-source platform for job sites. How to make it a business?

1) Sell professional support

This is how most open source software companies make money. This usually includes some horrendous subscription fee and guaranteed SLAs.

I never quite understood, why this works. My only explanation is, that many customers are comfortable with this, because this is what they are used to from buying proprietary software.

2) Sell a hosted version

This is classic SaaS. If you find a way to automate setup of instances of your software and offer this for an attractive price, this could appeal to people who shy away from the cost of running their own server and installing and maintaining the software.

I firmly believe that SaaS and Open Source do not contradict each other, but I only know one example that seems to be successful: http://www.teambox.com

3) Open Core

Sell a paid version of your software that has features not in the open source version. This is generally frowned upon by open source people, and rightfully so. At least you run the risk of destroying the healthy developer community you seem to have.

In your case, I would gather that you have a huge advantage when it comes to choosing the right approach. If you know who owns the hundreds of sites that already run your software then you already know some of your potential customers.

Talk to them and try to find out why they use your software, if they make any revenue of it, what their pain points are (if any) and what they would be willing to pay for.

it is difficult to give good advices without some data. As first thing, I would suggest to honestly ask your "customers" what would they pay for.

Maybe you can propose some new features and bug fixes only when you reach a certain ammount of donations (I mean, this is kind of the way the Diaspora guys made their money). If your target is developers, then to ask them money you may want to tell them how much time would they save with your new module or version. If a developer understands your new module would save him 3 hours of work, why shouldn't he pay for, let's say, as much as 1 hr of work is worth for him?

Finally, from my point of view, developers are easily the most price sensitive. So, another point would be: would it be possible to find another audience? Let's say small companies who may enjoy having their own job board?

At the moment your only likely to appeal to a very limited technical audience, if you made it so anyone could setup a job board you'd likely get a lot more customers.