I recently joined Dave's Garden (freealthough they have subscription services also). This site pulled me in because it carries a wealth of free information from plant profiles, to discussion forums and all kinds of articles.
What finally did it for me was their Plant & Seed Trading forum. This is something I've been wanting to try for the last few months but didn't quite know how to test the waters. One this site it's free to set up a Trade List, the process is slightly tedious but not overly complicated. You can start a Trade List from links in your profile, My Tools tab, or you click the Communities Tab and then scroll down to Plant and Seed Trading. There is a link on those pages to manage your own list by adding what you have to trade and what you want.
When adding a new plant first you must select whether you Have or Want it from a drop-down list. Then there are boxes to check what you have/want of that specific plant, the choices are Seeds, Plants, Roots Tubers and Bulbs, and Cuttings. To start adding plants you will need the scientific name, both the Genus and the specie which will be entered into different boxes. You may also add the variety name (the italisized one ex. Bright Lights variety of Cosmo) if you have a specific one in mind, the sytem will pickup the common name of the plant on it's own from the latin one. And finally there is the Special Notes box. If it's a plant you want you might repeat that you want seeds/cuttings and how many, perhaps also mention what you'd like in exchange. The same for things you have to trade.
After adding a plant it takes you to the list it's under (Have or Want). Under Plant Name it will usually have some options to check such as a choice of varieties/cultivars if it some are already listed on their site, and always the choice of "No Cultivar" (if you choose that only the typical common name will be used in the listing for searches). Be prepared to see variety names slightly different from the one you are looking for (happened to me for Lemon Basil) or to not see the specific one you're looking for at all. If your variety is not a choice, just add that to the Special Notes by editing the listing. A picture may automatically be added to the listing after you've chosen a cultivar from an existing one in their plant catalog, if you don't like it or they don't have one you may upload your own.
As I said before the system is not perfect and you will need to play with it a bit to get familiar, but it's worth a shot :)
If anyone's interested in trading with me here's my Trade Lists.