The Northern Forest Atlas Foundation, based in Lake Placid New York, has made three of their plant atlases available for free downloads from their website. Woody Plants of the Northern Forest, Mosses of the Northern Forest, and Sedges of the Northern Forest are all available now as PDFs.
Each document contains over 1400 pictures with an abundance of ecological information, and would be a very useful tool to assist with your plant identification needs.
A link to the Northern Forest Atlas can be found right here.
Bruce Grey Woodlands Association 
I don’t see any link to download
Hi Dirk. You’ll need to click on the link in my post, where it says “here”, and then click on the picture of the atlas that you would like to download, and then click on where it says “Download PDF”. However, it appears to be really slow right now, perhaps their server is being overwhelmed. There is a lot of data and these are large files.
For the link, see the very end of Kevin’s posting, where it says:
“A link to the Northern Forest Atlas can be found right here.”