Paper Jogger

Paper Joggers are machines that shake a stack of paper, called jogging, to line all the edges up into a neat stack. This typically also removes any static electricity between the papers, helping reduce double feeds/mis-feeds, as well as making registration more consistent within a printed stack. And it can help remove any dust and debris from the stack, so that they don't feed into the risograph and get stuck on the drum, potentially making marks on your prints.

Many risograph printers prioritize a paper jogger as a "big three" purchase for the setup of a printshop, along with the risograph machine itself and a stack cutter.

Tips for using the paper jogger

Many printers always jog before putting a stack of paper in the feed tray. New ream, already been printed on, partial ream, etc. This helps reduce double feeds and makes registration better.
When you put a jogged stack of paper into the risograph's feed tray, align bottom jogged edge to be the lead edge (as it feeds into the machine). Certain reams of paper are more unevenly cut than others, and it makes a big difference in registration.
Placing the paper jogger lives next to the feed tray of the riso can make it easier to jog before putting paper into the feed tray.
For text-based or low ink coverage stacks you can often jog them right out of the output tray without causing any smearing, and stack dry them jogged. There is less chance of the edges of the papers getting dinged/bent if they're jogged.
If there is time/space, you may want to wait before jogging a very inky stack until it's had time to dry... you can sett the paper tray wings to make the output tray's stack as neat as possible and then "stack dry" the papers without jogging, then jog after some time has passed.
Jog the stacks before cutting with a stack cutter to get the most precise and accurate cuts.
Some joggers orient the stack vertically, and some orient it horizontally. For vertically-oriented stacks, you can use your hand to "fan" the papers at the top as the paper is being jogged, to also help break up any static electricity or papers that are stuck together.

Considerations for purchase

In the US, used joggers cost between $15-200, with $100 being a common price. Joggers are very simple machines, mostly just a motor that vibrates a compartment that holds paper, so very old joggers are fine. Some joggers have a knob that varies the speed/strength of jogging, which can be useful since stacks of different papers and different inky-ness seem to be jogged most effectively at different settings.

Joggers come with different capacities for paper size; so you might want to get a jogger sized for the largest press sheet you typically print on. However, many of the vertically-oriented joggers can hold larger dimension papers than their tray is designed for. For example, if you hold the paper ream/stack with your hand a bit, or add extra cardboard/chipboard to the sides, you can jog an 11x17/A3 ream in a jogger designed for Letter/A4.