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).



Atlas-like websites on specific areas of mathematics
This [catalogue of mathematical datasets](https://mathdb.mathhub.info/) could be of some interest to you - at least some of the entries are atlas-like websites. It includes several of the websites mentioned above, and I'm slowly adding more to it.


