Must-have scripts/extensions/apps for beginner

It’s all just personal preference, really.
Shotty maintains a nice list of all the scripts: /t/The-New-And-Improved-List-Of-API-and-Third-Party-Apps/7694/1
I suggest you to read through it, and try out what you find interesting.

Some popular ones (based on the total number of GM installs): Stroke order, Dashboard Progress Plus, Override, Level Duration. Real Numbers, Ultimate Timeline, LevelUp Celebrator, Similar Kanji, Reorder Ultimate