These are some projects which share similarities with poliastro or which served as inspiration:
astropy: According to its website, “The Astropy Project is a community effort to develop a single core package for Astronomy in Python and foster interoperability between Python astronomy packages”. Not only does it provide important core features for poliastro like time and physical units handling, but also sets a high bar for code quality and documentation standards. A truly inspiring project.
Skyfield: Another Astronomy Python package focused on computing observations of planetary bodies and Earth satellites written by Brandon Rhodes. It is the successor of pyephem, also written by him, but skyfield is a pure Python package and provides a much cleaner API.
Plyades: A pioneering astrodynamics library written in Python by Helgee Eichhorn. Its clean and user-friendly API inspired me to completely refactor poliastro 0.2 so it could be much easier to use. It is now deprecated by the author, with Astrodynamics.jl being its successor (poliastro, too!)
orbital: Yet another orbital mechanics Python library written by Frazer McLean. It is very similar to poliastro (orbital plotting module was inspired by mine) but its internal structure is way smarter. It is more focused in plotting and it even provides 3D plots and animations.
orekit-python-wrapper: According to its website, “The Orekit python wrapper enables to use Orekit within a normal python environment”, using JCC. Orekit is a well-stablished, mature open source library for Astrodynamics written in Java strongly supported by several space agencies. The Python wrapper is developed by the Swedish Space Corporation.
beyond: A young flight dynamics library written in Python with a focus on developing “a simple API for space observations”. Some parts overlap with poliastro, but it also introduces many interesting features, and the examples look promising. Worth checking!
SpiceyPy: This Python library wraps the SPICE Toolkit, a huge software collection developed by NASA which offers advanced astrodynamics functionality. Among all the wrappers available on the Internet, at the time of writing this is the most advanced and well-maintained one, although there are others.