None that I am aware of. There are some excellent databases out there, but none used in the way of identifying a specific plant that you have seen, but is unknown to you. In fact, I find Google my best resource. If I know roughly to what genus or species, or if there is any characteristic description of the plant that can be use in the search, most of the time, I hit pay dirt by using the image search. Sites like the Royal Horticultural Society's website does have plant databases which allows you to enter specific plant description in your search. Most of the time, though, the search mechanics are not sensitive enough to give you a list precise enough to give you an answer.
Hello Liz, I think the original poster was looking for sites where someone could use keys or data queries to identify perennials (much like the European Garden Flora), as opposed to sites that help people identify plants. Agreed that Gardenweb is quite handy, as are the UBC forums, as are a few from this list of forums with regards to identifying specific plants.
