EDIT (May 10): This thread is no longer restrained to slopes. It now covers tileable terrain in general.
(Follow-up to this thread: http://www.cartographersguild.com/showthread.php?t=4655)
My D&D campaign being full of wilderness encounters (in plains), I was tired of having only rocks, bushes and trees to show. Unfortunately, I've never seen a well-done slope, so I started experimenting (see the above thread).
Now that I'm satisfied with the way I can get slopes to look like, I started producing a slope tileset. The first useable result is joined to this post in a zip file along with pictures of what it looks like. There's only 2-square long slopes for now, but I'm working on more.
Each tile of the set covers multiple squares, with squares 64 pixels wide. They're designed so that a small set of tiles can be rotated every which way and fit together seamlessly to make slopes as wide as desired. To save memory, gain versatility and have more variety, they have no texture of their own and must be added on top of a background texture (generic "plain" will do, as will a few others). This allows for really, really big maps, and scenarios (chase, exploration, string of fights) which would not be possible with a single, stand-alone map. Gametable crashes with 15-square by 15-square images and hardly handles their splitting in smaller images (hard limit of 10 MB of usable images). Maptool won't handle 30-square by 30-square pictures and slows down considerably as cumulative image size increases (it is, however, crazily efficient with textures and tiles). Thus the tileset.
While I had realism in mind at first, my priority is usability. Each square must show clearly what kind of terrain it is. If I indicate a slope (using up precious memory), it's a steep one that affects the players. Otherwise it's not shown. Seen from above, real-life slopes are only visible if the sun is low, and even then, not if the light hits them sideway. I show them by making their top lighter than their bottom (easily seen in the leftmost picture) and adding a subtle gradient shadow (even though the slope should make none). I also reinforce them with a pale grass/earth/rock texture; the combination of texture and lighting make the slope obvious no matter the zoom levels and clearly delimits which squares are part of it.
The complete tileset will have edges (denivellations between two squares), 1-square, 2-square and 4-square slopes and the necessary transitioning tiles. I will surely make a matching cliff tileset, and maybe combinations between the two (if the cliff gets higher, surely there's a slope somewhere...).