Here's some links to related sites that you should know about.

OFFICIAL SITES

ManiaPlanet.com
Access your player profile from here, create dedicated server logins, buy buddy slots, see who of your buddies are online and in what ManiaPlanet game, etc.

ManiaPlanet Official News
The official blog from the developers of TrackMania².

ManiaPlanet Forums
The official developers forums. Discuss all aspects of ManiaPlanet and give feedback to the developers. Notify fellow players of projects and competitions you are hosting. The ManiaPlanet forums support multiple languages.

ManiaPlanet Documentation
The Maniaplanet documentation is created to help using the tools of the Maniaplanet game operating system.

COMMUNITY SITES

ManiaPark
A community site dedicated to offering the best skins and 3d models of cars as well as environment mods. They also have their own forums for mod enthusiasts.

Dedimania
The community run online record service. See the best times on tracks or other stats.

ImagesPark
Need a place to host your custom signage or other TrackMania related images?

ManiaExchange
The site you're on now. There's also a subsite here for ShootMania maps.

TRACKMANIA FOREVER

TrackManiaForever.com
The official site for TrackMania United Forever and TrackMania Nations Forever.

TM-Exchange
The portal into a group of track sharing sites for the previous iterations of TrackMania, before TrackMania². TM-Exchange (TMX) is operated by the same people who run this site, Mania Exchange (MX).

TrackMania Carpark
Want a custom ride in TrackMania Forever? Look no further than this site with 100s of custom models, and countless more skins.

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More links will be added as needed

Updated February 16, 2019: Removed Player Page and MP Wiki. Renamed Trackmania 1 to Trackmania Forever.

Updated February 14, 2017: Removed Mania-Creative, Electronic Tournaments, TM Screenmania, and TM-Tube, since those sites have closed. Removed Planets Millions Cup since that competition has been discontinued.