I kinda dealt with this same demon a few years ago with some resources I downloaded. I wasn't sure how I was going to sort them, (it was mainly with 2D textures I found online). Luckily, I never really used them, and if I did, it was just for inspiration.
I do have plenty of downloaded "foliage" from free sites, but by the time I re-tweak them, they are barely the same item (not to mention I don't recall where I got them). In fact, I don't think I've even used any of them in my images, so they are just wasting space on a drive. This does not include the X-Frog free TG2 foliage, which is a dead giveaway in images.
So I guess what I am getting at is, its worth researching and creating your own stuff, while using other's creations as inspiration (or to show what is possible). Additionally, since procedurally created foliage, terrains, textures, etc are all 100% your own creation, I've gone that route. However, I am still considering buying some X-Frog models.
How's that for a ramble!?