Thematic maps are very useful when describing statistical data about places. The data, simply, becomes much easier to understand.
Cartographer is a JavaScript library for creating thematic maps on Google Maps which supports custom styling.
It is only a one JavaScript file that relies on the powerful Raphael.js vector-graphics library.

It supports 3 visualizing types:
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Chloropleth (color coded regions)
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Pie charts
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Point clusters
The areas on maps are mentioned as latitudes or regions (depending on the type) which means no hacking or manual calculations.
Every visualization type comes with different options like colors, opacity, stroke & more.
P.S. The library is currently on alpha state but pretty solid.
Requirements: RaphaelJS JavaScript Library
Compatibility: All Major Browsers
Map Icons Collection is a set of 600+ free placemark icons that can be used to mention almost anything on maps with style.
The set is categorized very well in the project's wiki pages like restaurants, culture, sports & much more.
Sizes of the icons are mostly 27*27px or 32*37px and they are all transparent PNG format.

Mapeed.AddressChooser is a Javascript script for creating an address form with a twist: it displays the address mentioned in Google Maps (can be used with another map provider by creating your mapping proxy) in real-time.

The script adds a behavior on HTML form, & doesn’t generate HTML markup.
It can also be plugged on any existing forms.
You can focus the map to the user’s city/country with the IP used (if mapping provider supports).
Mapeed.AddressChooser is well documented & has a nice demo with script.aculo.us autocomplete.
Requirements: No Requirements
Compatibility: All Major Browsers
uMaps is an universal ActionScript 3.0 mapping API.
This flexible API enables developers to build fully-customized solutions. The current version is integrated with OpenStreetMap and Microsoft Virtual Earth map data.

uMaps features:
- Support for custom map tiles and providers
- Flash / Flex versions
- KML Support
- FLA-Based Map Controls
- Marker Manager
- Advanced content layering
Using a universal API is very helpful as you can easily switch between mapping providers without updating the codes. As Google Maps API for Flash is released (see WRD post), it may be integrated into uMaps too and can make the product better.
This Flash mapping API is free for personal use and requires a license for commercial use.
Requirements: Flash 9+ AS 3.0, Flash Player 9+
Google released the Google Maps API for Flash which lets Flex and Flash developers embed Google Maps in Flash applications.
v
The API is smooth and fast. Flash graphics can be used for each tile layer, marker and info window. This opens up possibilities like dynamic shading, shadowing, animation, and video. When the user zooms the map, magnification changes happen smoothly and place names fade in.
The title is fun, I know (and I love Lords Of The Rings).
Mapstraction has kind of a similar logic. It provides a common API for the major mapping providers. This means, when you code with Mapstraction API, you can easily switch from Yahoo Maps to Google maps or else.
You can even use this power to create creative solutions like this syncronizing maps example:

Some features of this mapping API:
- Support for 9 major mapping providers
- Point, Line, Polygon support
- Image overlay
- GeoRSS and KML feed import
- Geocoding of addresses
- Driving directions
Requirements: No Requirements
Compatibility: All Major Browsers
Smoot is a tool, developed with xajax and Pear::DB for creating diagrams of icons and links between them.
Features include Google Maps and Google Earth integration, configurable tool palettes, and automated palette generation from a Google local query.

Smoot builds relations between objects with mouse clicks and the result can be stored easily. This nice tool can be a starting or an inspiration point for many projects like "online heritage services", "online diagram services" and much more.
Requirements: PHP, MySQL 5+
Google already has an API for Google Maps service which makes it possible to create interactive, real-data-driven maps.
DrasticMap, an AJAX based component, makes it easier to show data from a MySQL database in Google Maps.
This Google Maps component is offered in a PHP – JavaScript package called DrasticTools which also includes 2 other components, DrasticCloud (for easy tag cloud generation) & DrasticGrid (grid editing).

DrasticMap uses MooTools JavaScript framework for the interaction, which is included in the download package.
Requirements: MooTools JavaScript framework (included in the download)
Compatibility: All Major Browsers
Exhibit is a three-tier web application framework written in Javascript which makes it easier to show a few hundred records of data on maps, timelines, scatter plots, interactive tables, etc., without any programming knowledge.

In order to use Exhibit, writing a simple data file, and an HTML file that specifies how the data should be shown. Data + Presentation. That’s all.
Exhibit can be used as a web service like Google Maps without installing any software or source code can be downloaded to run it on your own web servers.
Requirements: No Requirements
Highlighting areas like countries, cities in a map image is a time consuming job.
Many alternative images need to be created for the mouse over effects & they must be linked to each other too.

Mapper.js allows you to add automatic area highlighting to image maps on your webpages (inc. export to SVG).
It works in all the major browsers – Mozilla Firefox 1.5+, Opera 9+, Safari and IE6+.
Requirements: Mozilla Firefox 1.5+, Opera 9+, Safari and IE6+