Dave Johnson seems concerned that Web 2.0 and Open Source seem mutually exclusive:

Web 2.0 is not about having cool software to install on your own personal web server, it’s about getting locked into services provided by and trusting your data to Web sites that you do not control. It doesn’t have to be that way, of course, and perhaps I’m exaggerating a bit just for fun (and hits). But maybe we need a list of “the best open source Web 2.0 software” – and it would include things like open source blog servers, wiki servers, photo galleries, content management systems, social bookmarking clones and etc.

I was talking about the benefit of Open Source in Web 2.0 earlier in “Working over the web”:

The great thing about using Open Source applications like OpenPsa is that you can choose between having it hosted for you, and running it yourself. At first you can get the hosted version, and get all the benefits of not having to hassle with backups, installations and other things. And if you later in your company’s growth path decide it is better to run the software yourself, you can easily switch to that model.

Actually you could even run a local OpenPsa installation on your Mac or Linux laptop and synchronize using Exorcist to achieve Notes-like replication.

Via Sam Ruby.