Data storage and manipulation
The StructArrays package provides an efficient implementation of the "array of structs to struct of arrays" transformation.
The packages JuliaDB and JuliaDBMeta add, on top of a "struct of arrays" container, the ability to perform standard data manipulation (groupreduce, groupby, join, stack, unstack, etc...) and a set of macros to simplify the API.
The ShiftedArrays package provides lazy shifted views of an array. It can be useful to store shifted views of a variable that is continuously recorded during an experiment, shifted relative to relevant events.
Visualizations and interactivity
Lately, I've been considering the exciting new high-performance plotting package Makie and wrote an API layer on top of it, AlgebraOfGraphics, to create data visualizations using the Makie infrastructure.
For building user-interfaces, I've contributed to the rewrite of a well-established Julia package, Interact, which now allows two-way communication between native HTML5 widgets and Julia to easily build web-apps.
The Remark package allows to create markdown-based presentations with Julia code.