I’ve keeping my eye on a project called Elisa a lot recently. It’s basically another attempt to produce a cross platform* media centre solution. At the moment its features are fairly limited, it’s essentially just some nice media playing software. It does have plugins to handle Youtube and Flickr though with lots of other stuff planned.
The main reason I’m interested in it though is that it feels likes it’s constantly making progress (and they make the progress very visible). They have weekly releases and every one actually has some cool new feature. It’s also written in Python which I think lowers the bar a little for community contributions (as opposed to C++/Qt required for MythTV).
For it to replace MythTV for me the obvious missing feature is, well, TV. Reading the forum suggests it’s quite low on the priority list for the moment (since it obviously requires a lot of work and other features are less likely to be released in the meantime just because the developers are busy). But, as I said at the beginning, it’s worth keeping an eye on.
* Not so long ago it only worked on Linux, now it supports Windows. It technically supports MacOS with some messing around but they’re aiming for proper support soon.