TITLE: Atlas-like websites on specific areas of mathematics
QUESTION [71 upvotes]: In this post, we look for the existing atlas-like websites providing well-presented classifications or database about some specific areas of mathematics. Here are some examples:  

GroupNames: https://people.maths.bris.ac.uk/~matyd/GroupNames


Finite groups of order ≤500, group names, extensions, presentations,
  properties and character tables.


Atlas of Finite Group Representations: http://brauer.maths.qmul.ac.uk/Atlas/v3/


This ATLAS of Group Representations has been prepared by Robert
  Wilson, Peter Walsh, Jonathan Tripp, Ibrahim Suleiman, Richard Parker,
  Simon Norton, Simon Nickerson, Steve Linton, John Bray, and Rachel
  Abbott (in reverse alphabetical order, because I'm fed up with always
  being last!).   It currently contains information (including 5215
  representations) on about 716 groups [mainly finite simple groups or almost simple].


Atlas of subgroup lattice of finite almost simple groups: http://homepages.ulb.ac.be/~dleemans/atlaslat/


This atlas contains all subgroup lattices of almost simple groups $G$
  such that  $S≤G≤Aut(S)$ and $S$ is a simple group of order less than 1
  million appearing in the Atlas of Finite Groups by Conway et al. Some
  simple groups and almost simple groups or order larger than 1 million
  have also been included, but not in a systematic way.


The L-functions and Modular Forms Database: https://www.lmfdb.org/


Welcome to the LMFDB, the database of L-functions, modular forms, and
  related objects. These pages are intended to be a modern handbook
  including tables, formulas, links, and references for L-functions and
  their underlying objects [like field extensions and polynomial Galois groups].  


The Inverse Symbolic Calculator:  https://isc.carma.newcastle.edu.au/


The Inverse Symbolic Calculator (ISC) uses a combination of lookup
  tables and integer relation algorithms in order to associate a closed
  form representation with a user-defined, truncated decimal expansion
  (written as a floating point expression). The lookup tables include a
  substantial data set compiled by S. Plouffe both before and during his
  period as an employee at CECM.

If you know such a website on any area of mathematics, please put it as an answer (with a short description).

REPLY [2 votes]: http://www.polychora.de/wiki/images/ has renders of cross-sections, vertex figures and their duals, and cellets for many (possibly all) of the 1849 uniform polychora known at the time it was created.