PostgreSQL: Just a relational database, and usually behind the heavy-hitters in terms of features. Mainly notable for at least being competitive with the big, commercial databases.
You may say this, but that's because you're not a DBA. PostgreSQL drives Skype, Pandora, Reddit, and IMDB for example. Working in the financial industry, we use it, and a few other companies I know of are converting away from SQL Server to PostgreSQL. Financial companies are especially prudent considering the liability concerns, and we do a metric assload of testing; we don't just convert for shits and giggles.
The PostgreSQL developer community is probably one of the best organized and responsive I've ever seen outside the Linux kernel. They have a major release almost yearly, and the 9.x branch is especially notable, since they're starting to include "enterprise" features. It's one of the few DBs that can regularly post equal or greater performance than Oracle, and it just keeps getting better. There are a number of reasons for this, but it all involves all the storage and memory architecture improvements they've been incorporating in the last few years.
Its popularity was never as high as MySQL because, as you said, MySQL isn't a real DB. It's easy to set up, and gets the job done without being quite as stripped-down as something like SQLite. For just whipping up a DB-driven website, it was dead simple, and with the popularity of PHP, so too did MySQL gain momentum.
And there's another very active project: PHP. Argue all you want about their design philosophy, but that language took the web by storm. Python and Ruby are similar in the regard to being highly active and groundbreaking open-source projects. Rails and Django are both huge sources of newer websites these days, and for good reason. Hell, even Drupal is being used by The Onion, and that's another huge Python community.
You could just as easily say Apache is "just a web server". Especially considering Lighttpd and NginX have both been outperforming it for years now. They may not count because they're not huge community projects, but they're more than viable and used by major sites. My company's site, for example, serves 120MB sustained traffic all day long serving a financial trading application, and we run both NginX and Apache on top of PostgreSQL. The DB alone serves 10,000 transactions per second (and can scale to about twice that on our hardware) just nicely at peak times.
What, exactly, does it take for a release to be considered "worthwhile"? Abandoning major commercial vendors? They're doing that. Scaling to huge usage? Reddit uses an entirely open-source stack including PostgreSQL, RabbitMQ, and Cassandra, and just broke 1.2B page views per month. Not just any page views, but fully threaded forums with a moderation system. I already mentioned Skype and Pandora. Sit up and take notice? They already started doing that.
The term "clone" is also pretty subjective. Lots of projects are spawned simultaneously, and the commercial product inevitably reaches the market first because of the paid developers that just work on it all day long. But when a FOSS project gets some momentum behind it, it really catches up in a big way.
FOSS projects individually have their warts. But the market is also littered with commercial software that is a clone of a clone of a clone, and is either out of business, or a product nobody wants. The aspect of being FOSS is not a differentiating factor in that regard. But the good software, both in the commercial and open-source world, lives on, and the really good examples are adopted with greater velocity as they mature. Criticize open-source all you want, but discounting the it as an also-ran is a giant mistake.