Pokémon Surfing the Pokéweb: A user's guide to the online world of Pokémon

[Disaster Area]

There's plenty of places on the internet serving a variety of purposes; this thread should make it easier to navigate.

Competitive Pokémon Communities (English)
Pokemon Perfect
Smogon
Nugget Bridge (VGC-focused community)

Internal Links
Pokemon Perfect's Analysis Index
Pokemon Perfect's Tiers

Pokemon Simulators
Pokemon Showdown
Pokemon Online
Pokebattle (currently awating re-release, no date set yet)

Pokemon Simulators (No longer used):
Shoddy Battle (formerly Pokemon Lab)
Netbattle (and Netbattle Supremacy)
Porygon's Big Show

Pokémon Damage Calculators
PS! Calc
Sulcalculator (a bit less easy to use, but the 'set hp' option makes it easier to perform calculations where more than one move is being used and when more than one Pokémon is dealing damage)
Defense EVs Calculator [Additional Link: Original Discussion Thread]

Linguaspheric Communities:
Netbattle Forum (italiano)
PokeSpain (español)
Greek Forum (Ελληνικό Φόρουμ)

News:
Serebii

Information:
Veekun (Best for most aspects of competitive Pokémon)
Bulbapedia (Slightly less reliable than Veekun but with a much broader scope)

Other Sites and Forums
TCoD (Fan-site / Forum)
BAM (Competitive Fakemon Project, run by Disaster Area)
Pokestadium (In particular, a great sprite resource)
PokéBeach
Azurilland (formerly Marriland)
RBY2K10 (No longer accessible)
PokéCommunity (Notable ROM-hacking community amongst other things)
Glitch City Laboratories
GameFAQs
Azure Heights
ATQ (now down; linked to Bulbapedia article)
Marble Palace
Technical Machine (the first and possibly best simulator AI)
The PokeMasters
upokecenter (links to an archived page of the list of RBY moves. The original site is no longer online)
/vp/ (4chan board) [May not be worksafe]
/r/pokemon (nothing noteworthy here for the competitive player)
hax.iimark.us
Team Oblivion (TO)
Aeturnus (a merger between ATQ and TO)

 

[GGFan]

German forums:
Pokemonexperte (created in 2000, Pokemonexperte was actually one of the largest Pokemon communities for quite a while until Smogon grew and most people went to Pokefans instead).
Pokefans (created in 2007, Pokefans quickly replaced Pokemonexperte as the competitive hub of the German circuit. Although it eventually lost that title to BisaBoard, Pokefans is still running today).
Bisaboard (I'm not sure when exactly Bisaboard was created; maybe in 2008. After Pokefans began to die in 2013-2014, Bisaboard reached the top of the mountain and is currently the most populated and active German community).

Other:
THE Alternative (a valuable relic for the competitive Pokemon historian, particularly RBY. THE Alternative was the first community dedicated solely to RBY).