Postgres-XC has been around for a while, it is primarily developed by NTT and EnterpriseDB. It has a small community but a dedicated engineering/hacker backing. Postgres-XC is interesting because it keeps reasonably up to date with the latest Postgres (1.0 is set to be based on 9.1 of PostgreSQL) but provides a shared nothing clustering architecture. This type of infrastructure is one of the holy grails of web based applications.
Should Postgres-XC deliver on its promises (hint: it does), you will be able to scale out (as opposed to up which Postgres already does extremely well) at an almost 1 to 1 ratio. This means that instead of having to purchase 2 large machines at 10-12k a piece you could purchase 4 machines at 1.5k a piece and achieve similar performance (theoretically, I need to test this). It also means that scaling out in the "cloud" will be easier.
I invite everyone interested in PostgreSQL to take a look at Postgres-XC. It is going to 1.0 soon and it needs community members to help flesh out the warts that haven't been found yet.
Another Postgres fork that has recently appeared is tPostgres. tPostgres (doesn't that look wrong at the beginning of a sentence?) is set to do to Microsoft SQL what EnterpriseDB did to Oracle, with one minor, small, interesting, exception: tPostgres is Open Source. Further Microsoft SQL is more in line with PostgreSQL in the types of workloads you usually see it performing. Imagine a tPostgres with Postgres-XC. Imagine an open source way to easily port Microsoft SQL apps to PostgreSQL.
Now don't get me wrong, the latest versions of Microsoft SQL are actually good products. Yes, I did just say that. However, they are not Open Source, they are expensive (comparatively) and let's get real, we want everyone to run Postgres.
Unfortunately tPostgres is only just announced and they are literally at the beginning of building their community but as it is being initiated by Denis Lussier (co-founder of EnterpriseDB), I imagine that he will come through with something very interesting indeed.