1.Wikipedia—https://en.wikipedia.org/
Wikipedia provides some informations that may not be that comprehensive in other sites . For example, cultivars and selections. I can find complete lists and descriptions about cultivars there.
2.Plant ID—http://brittlikesplants.blogspot.ca/
This is a cute small site that lists key informations of plants in a pretty clear way. It is easy to find what I need there.
3.Mabberley’s Plant-book: A Portable Dictionary of Plants, their Classifications, and Uses Hardcover – Jun 16 2008
This is kind of a classic book with dense informations. Good to start with looking for basic things like family name. It’s also a good source to look for uses of plants.