It's very easy, do like Mozilla - they do a fully open source project and then they get millions from a big search company. Oh. Wait. They aren't making money _because_ of the software, doh. :-)

On a more serious note, you can become an expert in that open source package and charge for services, although the model plays in a market that gets very competitive very quickly, so I totally get that you have to be able to add value with your own stuff that you license.