Not sure if you want trees, shrubs, or herbaceous plants. Think these are all geared to native plants, but maybe they'll help:

for trees
http://www.uwsp.edu/cnr/leaf/treeid.htm
http://www.cfl.scf.rncan.gc.ca/IMFEC-IDECF/home.html

for flowers
http://www.ct-botanical-society.org/galleries/galleryindex.html
http://www.missouriplants.com/

Also, especially if you can post a photograph, you try using GW's Name that Plant forum.

Here is a link that might be useful: Name that Plant forum

A Texas A & M Univ. site that has several hundred photos of Texas native herbaceous plants is listed below. Enter search parameters or click on "List all Plants" at bottom of page

For info on Texas native shrubs: http://aggie-horticulture.tamu.edu/ornamentals/nativeshrubs/indexscientific.htm
A site for trees is also available.

Another source of native plant photos, indexed by flower color and leaf arrangement, is: http://www.missouriplants.com/index.html
Rb

Here is a link that might be useful: TAMU Horticulture-Herbaceous Plants

Try the USDA at: http://plants.usda.gov

I think the U of Missouri has the best one but alas during an up date I lost all my bookmarked sites. Maybe someone else knows it.
Meanwhile i want to thank everyone for helping me get some more bookmarked sites. Really need them.
thanks
oakleif

Sorry loris, did,nt see your Missouri link the first time around. So double thanks to you.
oakleif

I know you asked for plant ID sites, but I thought you might be interested in this particular website, too. I often have trouble identifying reseeders and other seedlings I've grown (if I "forget" to tag my seed containers). This site is enormously helpful in identifying seedlings, and there is even a section of weed seedlings, too.

Susan

Here is a link that might be useful: Seedling Identification

I need to post one from my state...http://www.ces.ncsu.edu/depts/hort/consumer/factsheets/
This site keeps getting better and better. I also use the USDA site and find the distribution maps to be particularly useful.