HERE WeGo Map — Offline map with Android and iOS apps.
Maps.me — Maps for places all over the world can be downloaded for offline use. For iOS and Android.
GIS Corps — Coordinates short term, volunteer-based GIS services to underprivileged communities.
GroundTruth Initiative — Use digital media, mapping, and open data tools for ground-truthing data.
Carto — Browser-based mapping and analysis of location data.
Distillery — Simplify geoJSON to suit your needs.
[Esri Story Maps](organization, data, data collection, monitoring & evaluation) — Combine authoritative maps with narrative text, images, and multimedia content. They make it easy to harness the power of maps.
Field Papers — Print an atlas, record what you see in the world through crowdsourcing the maps, and upload the data to OpenStreetMap.
Google Fusion Tables — An experimental data visualization application to gather, visualize, and share data tables. Allows for maps, charts, Motion Chart a.k.a. GapMinder, etc.
Here Maps — Road-centric maps. Built by a consortium of car manufacturers — the offline apps are really great.
MapBox — Pick from a dozen beautifully designed maps and create an application from it in just a few seconds. Great for creating visually appealing maps from pre-existing or pre-cleaned data. Also has iOS, Android, and mobile web versions.
MapHub — Make your own map by adding points, lines, polygons or labels; allows you to import and export easily.
Mapshaper — Tools for editing geospatial vector data.
Natural Earth Data — Public domain map dataset available at 1:10m, 1:50m, and 1:110 million scales. Featuring tightly integrated vector and raster data.
OpenStreetMap — Open-source map created by community of mappers that contribute and maintain data about roads, trails, cafés, railway stations, and much more, all over the world. Check out the wiki for more information.
OpenStreetMap Directions — Google Maps alternative for directions (routing from point A to point B) using OSM.
Stamen Maps — Toner, terrain, and watercolor maps, free for the taking.
Vector Maps — Various formats and projections.
ESRI — The industry-standard in GIS software and solutions. The suite of ESRI tools is pretty expansive, ranging from consumer-level items like their smartphone apps all the way up to enterprise-level server platforms.
Mapbox Studio — Design studio to create stunning interactive maps (formerly TileMill).
QGIS — Free and open-source GIS software platform.
R — Software programming language and software environment for statistical computing and graphics.
Tableau — Data visualization and business intelligence software (with built-in mapping charts and widgets).
Climate Change and African Political Stability Map — Comparing climate change to political stability with an ESRI map (complete with controls and sliders).
David Rumsey Historical Map Collection — 80,000 maps and historical images from all over the world.
Demographic and Health Surveys — Analyzes and distributes survey results regarding health, nutrition, HIV for over 75 countries.
Your Vector Maps — Site with a bunch of links to vector-based maps and shapefiles of the whole Earth. Good for building things in Illustrator or Sketch or the web.
Free GIS Data — A list of links to over 300 sites providing freely available geographic datasets, all ready for loading into a GIS.
GeoPlatform — Shared and trusted geospatial data, services, and applications for use by the public and by government agencies and partners to meet their mission needs.
GIS Lounge — Introduction to using GIS data, find up to date and interesting types of raster and vector data and further resources to other downloading sites.
HealthMap — Real-time intelligence on a broad range of emerging infectious diseases for a diverse audience including libraries, local health departments, governments, and international travelers.
Malaria Atlas Project — Disseminates free, accurate and up-to-date information on malaria and associated topics, organized on a geographical basis.
Map Pron — Subreddit devoted to new/interesting maps. Great for inspiration and falling down rabbit holes.
MapGive — An initiative of the State Department's Humanitarian Information Unit. Several highly recommended video tutorials explaining the importance of OSM for humanitarian response and how to edit using the ID editor.
Mapillary — Crowd-street Street Views.
Open Educational Geography Resources — Wide range of free online geography and GIS courses, including 'Problem-Solving with GIS', 'Cartography and Visualization', 'Open Web Mapping', and 'Location Intelligence for Business'.
Poverty Net — Resource site for measuring and analyzing poverty - includes 'Poverty Mapping' section.
UNDP Projects — Detailed information on the UNDP's 6,000+ development projects in 177 countries and territories worldwide. Browse the summaries, click a filter on the right, or search through the full list of projects.
What3words — A universal addressing system based on a 3mx3m global grid.
WorldPop — Open access archive of spatial demographic datasets for Central and South America, Africa and Asia to support development, disaster response and health applications.
Evidence Gap Maps — Evidence gap maps (EGMs) consolidate what we know about what works in particular development sectors or thematic areas. It's very cool and useful, but only available for a limited number of topics relevant to Youth, CED, HE, and ED.
Radiant.Earth — Open geospatial data for positive global impact, and improved decision-making.
GeoNames — Global geo datasets.
Digital Democracy — Open source, offline map editor.
Global Forest Watch — Interactive online forest monitoring and alert system that provides satellite and other layers of datasets.
Digital Globe — High-resolution satellite imagery.