If you're a keener in certain fields and want excellent feedback then I like:

Alpine-L or:
Bulb-L

These can be found by entering them in a Google search (or any search). There's a half dozen -L sites, all specialized in a topic.

Garden buddies

www.forestfarm.com - most awesome, they sell everything.

and www.mountainmaples.com - an outstanding collection of japanese maples for all climates.

Also the gardens alive website sells all organic stuff. I always order their BT for my japanese beetle problem but they have all sorts of cool stuff.

One of my favorite sites is monrovianursery.com. Whenever I am designing another bed, I research plants through this website. Only problem - many plants they have aren't available in my area, so I have to find the next best thing. Try it, you'll like it!