There's a bunch of open source databases that have fancy features like this now, is there much sharing of code or implementation ideas between the projects? Did you look at the source of for example Mongo or PostGIS, or is it mainly implementing algorithms from papers?
- Runs in browser, for client-side indexing.
- Based off of tiles, instead of s2's 6-faced-cube thing. This allows it to be used with a lot of well established tools and mathematical formulas. Also much easier to reason about IMHO.
- Indexes points about twice as fast at the moment.
I also use s2, and it really is terrific. There are many cases where s2 is the superior choice (mostly due to its hilburt curve indexing scheme).
Proj4 is for working with projections. It comes with a handy command line tool that lets you easily perform chores like converting between coordinate systems/projects.
GEOS is a C++ library (with a C API on top) that's actually a port of a Java library called JTS. It's used for expressing structured geometry/geography primitives and performing calculations on them, as well as encoding/decoding them in the WKT/WKB (text and binary respectively) formats (the PostGIS "geometry" data type). All the geometry testing and manipulation functions, eg. ST_Intersects(), use GEOS.
GEOS actually comes with support for a spatial index called STR, or STRtree, but I believe it's not used by PostGIS. Instead, PostGIS uses Postgres' own GiST, which is a generalization of R-trees that perform better in a relational environment.