Development Tools
Force.com IDE: The Force.com IDE allows developers a full-featured, Eclipse-based coding environment, with capabilities like code completion, version control, collaborative development, and project sharing.
Force.com Explorer 
: This handy and lightweight .NET 2.0-based tool lets developers browse their schema, custom objects and fields, and build and test SOQL queries. (Source included).
Force.com Toolkit for Adobe AIR and Flex: Build Force.com apps with rich, dynamic Flex interfaces and offline AIR functionility using this toolkit.
Data Tools
Data Loader: The Data Loader helps you to perform bulk operations on your Force.com.com: insert, update, upsert and delete data, as well as export data from Force.com objects. You can use .csv files or relational databases as the source or target for this data movement.
Members:Force.com Excel Connector : This Excel plugin lets you access and update your data directly from Microsoft Excel, allowing easy reporting, mass updated and cleansing of any Apex-based data, including custom objects. (Source provided.)
Email Tools
Email To Case Toolkit : The Email-To-Case Agent (AKA Email2Case) is a sample implementation that pulls emails from your mail server and uses the Apex API to create new Cases or append Cases. Use this to enable Email-To-Case.
Telephony Tools
Force.com Connect CTI Toolkit : The Force.com Connect CTI Toolkit allows partners to connect nearly any telephony API to Force.com. It gives Salesforce.com customers the ability to have screen pops, click-to-dial, and automatic call logging directly from the Force.com web interface.
Community Contributed Tools
The section below lists tools created by the DFC community to support a variety of languages and use cases.
Ajax Tools : This package contains a collection of tools for developers working with AJAX toolkit to build s-controls. Included are: A syntax-highlighting editor that can load/save s-controls directly from your salesforce.com account ; extensive code samples for the toolkit, dojo, and the UI in a handy code browser; a javascript shell with completion and input history -- very handy for trying commands against the API; and a SOQL Explorer for browsing your data schema and building SOQL queries.
Scripting Tools: This offering lets you use the AJAX Toolkit and javascript as a scripting language from Windows via integration with WSH (Windows Scripting Host). With this package, you can write and run batch scripts to do common and/or bulk tasks using the Force.com API using nothing more than a simple text editor (no IDE, no compiling, etc).
Mass Update Anything: This handy cross-platform utility makes bulk updating or deleting records fast and easy. It's like a cross between the Excel Connector and Force.com Explorer that simplifies these tasks.
Workbench : A web-based application that gives administrators on-demand access to useful tools to manage their salesforce.com organization. Combining the power of both the Apex Data Loader with the Force.com Explorer, the Workbench can insert, upsert, update, export, delete, undelete, and purge data as well as describe any object in your salesforce.com organization
Document Generation : A web service providing PDF generation from any data in salesforce.com. An extension to Print Anything.
: This handy and lightweight .NET 2.0-based tool lets developers browse their schema, custom objects and fields, and build and test SOQL queries. (Source included).