These are links to some resources I commonly use. Many are referenced elsewhere in my posts but I put them here for convenience. I will update it as often as I can based upon my own whim. I will not respond to requests to add items to this page.

Reference:
GeoJSON

SQLite/SpatiaLite:
SQLite - Very lightweight, yet powerful, database engine.
SQLite Data Browser - A nice, simple GUI for SQLite.
System.Data.SQLite - .Net data provider for SQLite.
SpatiaLite - Spatial data support for SQLite

PostgreSQL/PostGIS:
PostgreSQL - Powerful open-source RDBMS
PostGIS - Spatial data support for PostgreSQL
PostGIS In Action - A great book about PostGIS

ESRI Stuff:
ESRI Resource Center (9.x)
ESRI Resource Center (10.x)
ArcGIS API for Silverlight/WPF (1.x)
ArcGIS API for Silverlight/WPF (2.x)

Developer Stuff:
Shapely - Open-source Python library for geometry processing
SharpMap - Open-source .Net mapping library
RssReader - C# component for handling RSS feeds
SharpZipLib - Open-source .Net component for handling zip files
DotNetZip - Another open-source .Net component for handling zip files
mod_aspdotnet - Runs ASP.Net under Apache on Windows
SilverlightRibbon - FOSS (New BSD) implementation of a ribbon control in Silverlight. I’ve been pretty productive with it.
WPF Application for Importing ESRI Shapefiles - An oldie but a goodie. Shows using dispatcher and delegates to incrementally display shapes as they are loaded.
Menu and Context Menu for Silverlight 4.0 - A nice context menu control for Silverlight. I have used this on a couple of projects.

Mapping/GIS Stuff:
Spatial references, coordinate systems, projections, datums, ellipsoids ? confusing? - A very good, succinct post by Morten explaining coordinate systems. I direct new coworkers to this regularly.
Accurate Distance Calculations in UTM Projections - Another good post from Morten. I also point developers to this a lot. It explains things much more succinctly than I can.
Tiles ? la Google Maps: Coordinates, Tile Bounds and Projection - Ever wonder what the tile X and Y is for a given location? This site lets you find out in TMS, Quadtree, or Google Maps. Very useful for testing and debugging.
Gaia Geospatial Platform - From the Carbon Project, I have found Gaia indispensible for debugging WxS calls. This is always one of the first things I install on a new machine. It also has a Mono build for Linux users.
CartoDB - Hosted spatial data storage using PostGIS. Features nice HTTP + JSON SQL and Map APIs.

Useful Sites:
JSON Format - A nice online resource for quickly prettifying a block of JSON text.
GeoJSON Lint - A great tool by Jason Sanford for validating GeoJSON.