One of the most exciting developments in 9.1 will be the new CREATE EXTENSION feature that Dimitri Fontaine has been designing. Dimitri has been working on them for some time now, proposing them in detail at the 2010 Developer Meeting. Recently he's been coding them and the first patch arrived a few days ago. Reviews should happen in November, more than enough time to happen for the next release.
What do extensions do? Make it easier to add and remove plugins, datatypes, functions etc with minimum fuss and without needing to compile things yourself. It turns out that there's a great many aspects to this and much more complex than I've made it sound.
Dimitri has himself designed a number of very cool add-ons for PostgreSQL and its from those that he's gained insight into what's required. I've been trying to follow it myself, but I'm a bit lost on some aspects of it just yet.
Anyway, looks like there will be a few websites and repositories from which you can download extensions, so the hope is that it will be much easier to both obtain and install add-in components in future versions of PostgreSQL.
We're especially lucky that funding has been made available from the 4CaaST project, a Cloud Computing project funded by the European Union (under FP7). More on that another time.