I know there is the Tileset repository and the Graphics set repository in the The Dwarf Fortress Wiki, but these are incomplete and very outdated. I think creating an easily accessible thread where people can simply post a screenshot of their Dwarf Fortress instance (preferably part of the menu + part of a busy section of your fortress) with the given tile/graphic set would make things easier.
If this actually turns out to be a good idea, I'll try to keep the very first post of the tread (this one) updated with what the users here have posted and assemble it into some kind of big gallery. Some rules need to be set though: Please explicitly state the type of set (tile/graphics), its name and version at the time of the screenshot and if available an URL to its website/forum. A good and reliable image hosting service is imgur.
Tileset
I'm not a moderator so I can't really enforce this, but if you could please follow this template that would be perfect, else we will have incomplete/erroneous entries, image that distort the forum layout etc. The gallery link is optional and only needed if you want to showcase more than one image. Duplicate entries are also allowed if you think that you can showcase the set better/in a different way, I'll do my best to extend the gallery then accordingly.
How are they outdated? The graphics set repository might be, but the tileset repository is not. If people want to upload another one, they can, the wiki is much easier for everyone to keep updated than posting in a thread hoping for the original poster to update. I updated it to add mine, for instance. Plus, the sheer amount of tilesets available would quickly make the thread unwieldy.
If you feel graphics sets, tilesets, or screenshots are missing, then add them to the wiki. Plus, we already have a stickied thread linking to popular tilesets and graphics sets. If you feel some are missing, feel free to post in there with your link request.
All that being said, if you really want to use my screenshots to preview mine, go right ahead.