Is there a tool / program that will allow one person to manage several multiple Steemit accounts?
I have seen some users who are actually managing several accounts simultaneously. I'm talking about a single person controlling about 10s, 20s, to even 100s. What makes it possible for them to do it?

When you say manage I assume you mean posting, transfers, etc. I wouldn't imagine that all of these area being controlled manually, it's very likely that is some automated processes (bots) that follow a main account or are programmed to perform certain actions. If you mention a specific function surrounding what they're doing I would be able to explain more about how it works :)

In terms of being able to do this manually, let's say for 2 or 3 accounts, this could be achieved on any front end by simply logging in and out, however some apps do make this easier, for example eSteem surfer allows you to quickly switch between accounts, you can see this in the image below:



Firstly, Maybe they use http://steem.supply website. You can add multiple accounts and check them detaily.

Secondly, you can use vessel (https://github.com/aaroncox/vessel) tool that is a steem desktop wallet. You learn about everything about below link :