Julia currently has a bit over 5000 registered packages at the moment of writing. To search among these packages, there are a few tools:
JuliaHub, or JuliaPackages, or JuliaObserver - web interface to search for packages.
Julia.jl - a curated list of packages by field.
Furthermore, there are a few notable packages, well-known on the community (or niche community), or that I thought were interesting, that I'll list below.
BioJulia organization: Bioinformatics infrastructure.
CSV: Utility library for working with CSV and other delimited files.
Dash: Julia interface to the Dash ecosystem.
DataFrames: Tabular data
DifferentialEquations: (part of SciML) Suite for high-performance solvers of differential equations.
DrWatson: Scientific project assistant.
EcoJulia organization: Ecological research.
Flux: Neural Network library - from their website: Relax! Flux is the ML library that doesn't make you tensor.
Franklin: Static site generator. Runs and shows the output of Julia code, so it's very handy for tutorials, like this one.
Genie: Julia full-stack MVC web framework.
HTTP: HTTP client and server.
JuliaSmoothOptimizers organization: Packages for nonlinear optimization research.
JuMP: Modeling language for mathematical optimization.
MLJ: Machine learning framework.
Optim: Traditional optimizer.
SciML organization: NumFOCUS sponsored organization with packages for scientific machine learning.
YAO: Quantum algorithm design for humans.