First comes the site and then optionally a topic where the discussion is, if there is something you need to know, do it there

Installing Guides for new graphics
(How to?) How to install new graphics on TTDPatch
(For more general install guides and questions about TTDPatch in General:
Where do I get help? HERE! - Wiki, Answers, Installing guide)
Managing GRF files
Graphic Manager ( topic )
Vehicles translate tool
bug tracker
Editing and making new Graphics
GRFCodec ( topic, download )
GRF Wizard ( topic )
New Graphics Specs
LibGrf
Palette documentation
GRF Creator
WinGrf
graphics tutorials (Backup: graphics tutorials backup)
sprite editor
GRFmaker
NML topic | NML documentation
Converting Graphics
GRDTOGRF (EXE > GRF aswell)
Win-DOS Conversion
Graphics Sites
GRFCrawler - Search for Graphics ( topic )
TTD Urban Renewal ( topic )
George's New Graphics ( topic)
more to be added...
Have you made a tool yourself, post below here and I'll add it to the list, then I'll delete your post.
No offtopic here

Update tracker
2010-12-28: added links to the NML topic and documentation (Hyronymus)
2010-12-27: Removed links to Dinges' graphics site, Michael Blunck's graphics site and the Total Town Replacement graphics site (all the associated graphics remain available in grfcrawler) (Hyronymus)
2010-04-01: Added link to GRFCodec nightlies (orudge)
2009-04-15: Added GrfAuthor (Szappy / Open source)
2006-04-25: Fixed newgrf, tutorial and George's links, added NFORenum (patchman)
2005-08-27: Added new Install Guide Section
2005-01-18: Added Grf Crawler (eis_os)
2004-12-04: Added sprite editor (Szappy)
2004-12-04: Added Total Town Replacement Set (TTRS) (Zimmlock)
2004-12-04: Added graphics tutorials (Purno)
2004-12-04: Added bug tracker (ChrisCF)
2004-06-03: Added LibGrf location
2004-02-18: Fixed newgrf.txt file location
2004-02-10: Added palette doc and GRF Creator (orudge)
2003-11-12: Added GRDTOGRF (eis_os)
2003-10-23: Added a topic for ChrisCF's LibGrf
2003-10-23: Added Vehicle names Translate tool
2003-10-23: Added George's site