Plants are all around us, but we often take them for granted or pass the mover in favor of life-forms more similar to ourselves. We pay the most attention to them when they flower or provide some specific resource we need (timber, spices, food, etc). The vast majority of plants in the world are unused or infrequently used by us, but that does not lessen their importance and the roles they each play in maintaining a healthy ecosystem.
This page has a slowly growing set of plant-based resources: ID sources, geospatial, and at least one temporal resource.
Please feel free to suggest additional ones.
California Buckwheat (Eriogonum fasciculatum)
Adventures in Field Botany/Illustrated Glossary
http://fieldbotany.pbworks.com/w/page/5987187/Illustrated-Glossary
A nice illustrated collection of field botany terms and plant characteristics.
AgroAtlas
http://www.agroatlas.ru/
Huge amount of free agriculturally focused ecology, species, and physical features data in map and GIS form provided by both the USDA and a variety of Russian organizations.
Biota of North America Program
http://www.bonap.org/
Phytogeographic and Botanical databases for vascular plats all through the US with many incredible maps. Especially the species Density Gradient maps indicating relative plant diversity across the US.
Botanical Society of America Image Collection
http://images.botany.org/
Searchable image archive of plants, pollen, fungi, lichens, plant geography, and more.
BRIT Digital Herbarium
http://atrium.brit.org/index.php
Botanical Research Institute of Texas (BRIT) plant data library and virtual herbarium.
Flora of Italy
http://luirig.altervista.org/
Searchable image archive of plants for Italy, includes scientific name search, region search, botany cards, and a checklist. Site is in Italian.
Forest Service (USDA) Fire Effects Information Home Page
http://www.feis-crs.org/feis/faces/index.xhtml
This is an excellent and tremendously underutilized database of detailed plant and animal information provided by the USDA Forest Service. The ostensible purpose is to provide fire ecology information, but the level of detail and information, especially for economically important species, goes far beyond the fire focus. The database is both searchable and browsable.
International Plant Names Index
http://www.ipni.org/index.html
The International Plant Names Index is a database of the names and associated basic bibliographical details of seed plants, ferns and lycophytes. The ultimate goal is to eliminate the need for repeated reference to primary sources for basic bibliographic information about plant names.
Internet Directory for Botany
http://www.ou.edu/cas/botany-micro/idb-alpha/botany.html
Alphabetical list of online botanical resources. A fantastic resource if you’re looking for country or region specific information. The search function does not seem to work, so it’s slow going searching through for organizations.
Natural Resources Conservation Service (NRCS USDA) Plants Database
https://plants.usda.gov/java/
Standardized information about the vascular plants, mosses, liverworts, hornworts, and lichens of the U.S. and its territories.
Pineywoods Plants Digital Gallery
http://jvk.ddns.net/jvk/PineywoodsPlants/index.html
List and digital library of plants for the Pinewoods region of Eastern Texas.
Pollen Viewer
https://www.ncdc.noaa.gov/cdo/f?p=519:1:0::::P1_STUDY_ID:5973
Animated maps of species specific vegetation changes over the last 21,000 years in North America, courtesy of NOAA
Tropical Native Species Reforestation Information Clearinghouse (TRIC)
http://reforestation.elti.org/
A database of information and resources geared toward reforestation of tropical regions. Searchable by geographic region, resource types, countries, ecosystems, subjects, and more. Great literature list with an easily searchable free literature section as well.
Tropicos
http://www.tropicos.org/Home.aspx
A database of nomenclatural, bibliographic, and specimen data accumulated over the last 30 years.
Writing for Nature 
I have a few more resources you might want to check out and add to your growing list. There are plenty more but here are a few that we have used often:
http://src.sfasu.edu/~jvk/PineywoodsPlants/index.html (Pineywoods Plants Digital Gallery)
http://www.ipni.org/index.html (International Plant Names Index)
http://www.tropicos.org/Home.aspx (Tropicos)
http://plants.usda.gov/java/ (Plants Database)
http://atrium.brit.org/index.php (Atrium: Biodiversity Information System)
Oh, excellent. Thank you. I need to update this page soon and I will most definitely add these links.
My Pineywoods Plants website is now here: http://jvk.ddns.net/jvk/PineywoodsPlants/index.html I lost the server at src.sfasu.edu. I am trying to get a redirect so the old links work but Pineywoods Plants is back online at the above address!