I had a quick search for mention of conveyor belts. Didn't see any.
[edit: Which is strange, because I've since seen them as a voting item on the "A new round of suggested voting has started" thread, which seems to indicate that a search should have found them. Maybe I typoed. Okay, then, I'll see if I can read up on past suggestions, and feel free to ignore the naive mutterings that follow. But I'll let them stand for all to see, regardless...]
Now, conveyor belts I think are 1900s tech (suspended bucket systems are older), but I don't see them as beyond the artifice of a decent dwarven industrial society. I envisage a mechanism (or multiple/divisor thereof) needed per square and a power source, created with a N-S/E-W direction and from/to directionality like a pump (or just a series of 1x2 units that can be linked end-to-end). The input end could perhaps be given a stockpile-like item preference for dwarves to place items on them, items falling off of the end to pile up, or taken along a new route where there is another belt at that spot. Alternately, only one item (especially of bulky items) falls off of the end and the rest of the belt ends up with one item occupying each square until there is no space left, movement reoccurring when the resource is retrieved.
(This could highly mess up resource accessibility, as well as overpower what was originally a haulage option, if you want to know the main reasons why I think it's an idea to reject.)
However, if a belt driven by the existing power sources (waterwheels, windmills) a variant of the pump (dwarf-powered) or the speculative ones mentioned in this thread (steam-power, 'lectrickery) isn't practical, there's always a "roller gradient" or plain 'chute' solution. Perhaps limited in length (to seven blocks or so, mirroring the same sort of height differences that water has, as the item goes between squares?) and no ability to drop off the end onto another such conveyancing device (unless you're fans of Escher-like constructs that defy the z-axis).
Otherwise, a variant of the current pump system would use the Archimedes Screw principle to convey bulk items (sand and seed, but not cages or rock blocks) in some way, instead of water.
Again, if any of these are considered too modern (despite historic precursors and the fact that dwarven development is already proven to be fairly sophisticated), a gravity and/or powered binary tram system. Two (or 2x) squares wide, carts of 1 (or 'x') width on two tracks with a rope (or several) linking them, sending them back and forth between two points with goods (or dwarves?) on board. A not-quite-like-system was around in 600BC in Greece or... sometime in the middle of the first millennia (CE) for China to develop a more similar system, if you were wanting me to provide (loose) reference dates.
Sorry. Got a bit carried away. What I meant to say was just "Hey, has anybody mentioned conveyor belts yet?", but my fingers have a life of their own, sometimes.