Popular current bottom-feeders:
REO (Aluminum Mini, Aluminum Grand, original Wood Grand, and newer Woodvil)
Phidias (wood)
Ali'i (wood)
Journeyman (wood)
Wetbox (plastic)
Old Goat/Stu's (plastic)
Red Sky Scallywag (wood)
Monkeyboxx (wood)
Southpaw (wood)
Sweet Vapes (wood)
Others may be available, but those are the most popular, as far as I know (if I've omitted any, I apologize).
The various bottom-feeders share common features: They all use an internal bottle for holding juice (usually 3ml or 6ml) that can be refilled or swapped out. They all feed the juice upwards through a hole in the center of the atty/carto connector via tubing (rigid stainless steel, flexible vinyl/silicone, or a syringe needle) into the bottom of the atty/carto (thus, "bottom-feeder"). Juice is "delivered" by squeezing the internal feed bottle (or "squonking" in the parlance of bottom-feeders) either manually through a cutout in the mod or by pressing a button.
From there, differences abound. Each bottom-feeder uses a slightly different design for feeding. The "boxes" vary from barebones (2AA plastic Old Goat and Wetbox) through artisan hand-crafted furniture-grade woods (many), to rare and astonishing figured woods (again, many). At least one (Sweet Vapes) is not a "box" at all, but a variation on a pistol-grip shape.
Sizes of the finished units vary widely, from small (Old Goat, Wetbox, REO mini) through medium (Red Sky), to relatively substantial (Phidias, REO Grand, Ali'i, Southpaw), and finally to quite a handful (Monkeyboxx, Sweet Vapes).
Almost any combination of batteries and power options is available from one mod or another: 17500 and 18650 batteries, or a pair of 123a; with voltages of 3.7, 5 regulated, 6, 7.4, and variable.
Prices vary widely, of course. At the 2AA plastic box low-end, we're talking $40-$60. From there, the wood boxes and REOs leap from $100 on up, with the full bells-and-whistles rare-figured-wood and VV options catapulting the price above $200.
All the various bottom-feeder custom mods have huge followings of devoted fans who wax euphoric about their chosen model by filling hundreds of pages of threads here on ECF. That said, what is less obvious is that not everyone ends up bonding to his or her bottom-feeder, despite their seemingly overwhelming popularity.
To cite just one possible downside: Juice leaks are an inevitable fact of life in the world of vaping, whatever style PV one uses, but bottom-feeders increase the likelihood of leaking for varying reasons---imperfect feed systems, overzealous squonking, caps to feed bottles not screwed on tightly enough, etc.). However inconvenient wiping off an occasional external overflow may be, opening one's bottom-feeder to find that juice has leaked all over the inside can be truly aggravating and a PITA to clean up. I'm not implying that this happens for everyone or necessarily often (so please don't flame me), but it can and does occur. I know, because I own three bottom-feeders.
Although bottom-feeders replicate the clean, rich flavor of atty-dripping, arguably in a more convenient way, some people just prefer the ritual of dripping. Others like the "simplicity" of cartomizers with their less-frequent topping off. Still others love tanks in any of their numerous sizes and styles: straight CE2s, MAP tanks, syringe mods, Ego-T, etc.
Bottom-feeders are ideal for people who tend to like an all-day vape of a single flavor; they are less perfect for those who flit from one juice to another every five minutes (as I do).
All in all, this particular niche within vaping is yet another example of the mind-boggling diversity of products available here at the final feverish flowering of the global consumer marketplace during the twilight years of modern industrial society. Anything one wants and/or can afford is within reach---at least, for a little while longer, until the Big Collapse hits. So stock up now, kids, while all this stuff is easy to get and fairly cheap.