The first tool everybody should know is https://steemworld.org This site lets you keep track of all your account operations in real time. Just add your username /@abcdef.... to the end of the link and it will give you all the info. Followers, voting power, incoming rewards, votes...all in real time. So handy.

This one isn't so much as an app. https://partiko.app/ Partiko is the best new thing that i've seen on steemit lately and is an app to access the site but one which has a full set of notifications for your profile as well as being smooth and easy to use. I've found it so much more enjoyable to use steemit since i have downloaded the app.

The last one is http://www.steemstyle.com/ which is a markdown editor. For those of use who aren't very computer literate, this site will format your text in a quick and easy manner to add style and definition to your posts. It has a simple range of tools to format headers, paragraphs, italics, ect...

For me personally these would be three of my most essential tools while i am on the site and i've no doubt that you will find them just as invaluable as well. Enjoy.