--> A little while ago I stumbled across this free online textbook, "Fundamentals of Physical Geography" by two researchers at the University of British Columbia.
The FUNDAMENTALS OF PHYSICAL GEOGRAPHY
I've found it to be an excellent resource for worldbuilding, and it's written in a way that makes it easy to grasp the concepts even if (like me) you aren't that proficient at science and math. It takes you all the way from the origins of the universe and the earth through to climates, ecosystems and the evolution of various flora and fauna. The nice thing from a worldbuilding perspective is how it breaks out the workings of various systems into separate sections, so if you need to work on rivers for instance, it's easy to find the info. The diagrams and animations are also excellent.
It's free and available as on online resource and a downloadable ebook. There's also a glossary and some other things. I highly recommend it!
--> My other favorite resource, geared specifically towards climate workings, is the Climate Cookbook.
The Climate Cookbook
Geoff has a page on creating planets which is helpful, but this section I love. It's a step-by-step guide to building working climates for fictional worlds. I use it together with the Fundamentals ebook as a backup reference.
--> For a source of inspiration, I like Chris Wayan's Planetocopia page.
THE WORLD DREAM BANK: PLANETOCOPIA
Chris has built a multitude of fictional planets and alternate earths, each exploring different issues in worldbuilding. Maps, information on the native species and various sentient peoples is available for each world. It's a fun and helpful site to explore.