There is also The Encyclopedia of Life which I saw a while ago. It's scope is more broad, and IIRC, it is more oriented toward professional botanists, whereas The Plant Encyclopedia is suitable for laymen too.
Interesting site. Saw it today via a search for some plants. Apart from info about plants, the site is interesting because of the tech it uses - something like MediaWiki (that powers Wikipedia) but with some changes.
Note to HN readers: I've deleted the post from my blog, because, though this did not happen initially (when I wrote the post), a day later the link to the site was not resolving correctly and I was getting a warning message from my browser.
Not sure of the reason, since it worked the day before. Maybe their certificate expired.