Calculator for 3D Printers
http://www.prusaprinters.org/calculator/
This tool is great for steps per mm, acceleration and Optimal layer height.
Convert pictures to STL format
https://www.selva3d.com/
You can sign up for free and get a low-quality STL for free. You can get higher-quality STL's for a price.
Hand tools
https://www.amazon.com/3D-Print-Removal-Tool-Professional/dp/B01AJI1M04/ref=sr
A good scraping tool will help you get your 3d prints off the heat bed. Any scrapper like this one would do the trick
https://www.amazon.com/Repkord-Precision-Drilling-multi-size-compatible/dp/B01M0G0HQ8/ref=sr
A drill bit kit like this one will help you clean out your extruder when it gets clogged.
https://www.amazon.com/Hakko-TRR-58-G-Hardened-Construction-21-Degree/dp/B00FZPL6NU/ref=sr
Flush cutters can really help with trimming pieces off your 3d prints.
https://www.amazon.com/Long-Needle-Pliers-Jewelers-Electrical/dp/B000RB5YBC/ref=sr
Needle nose pliers can help with pulling supports of your 3d print.
Heated bed adhesion
https://www.amazon.com/Duck-240180-Release-Painters-Inches/dp/B0054P49YE/ref=sr
painters tape can help stick PLA to a non-heated bed but not ABS.
https://www.amazon.com/Elmers-Purpose-School-Washable-0-24-ounce/dp/B001E69WBW/ref=sr
Elmers glue sticks also work with PLA and sometimes works with ABS on a heated bed.
http://www.instructables.com/id/ABS-Juice-and-Slurry-Getting-your-Prints-to-Stick/
ABS juice works great with ABS on a heated bed.