If you use something that is great and don't see it listed here, please let me know!
.NET Essentials
.NET Framework SDK version 2.0
You need this to develop any .NET 2.0 applications, including ASP.NET 2.0.
.NET Framework SDK version 1.1
You need this to develop any .NET applications, including ASP.NET.
SQL Server 2005 Express Edition
Full power of the SQL Server 2005 engine with the ability to create databases up to 4GB is size.
IDE / Code Editors
Visual Web Developer 2005 Express Edition
Free edition of Visual Studio for developing Web application with ASP.NET 2.0.
ASP.NET Web Matrix
Free WYSIWYG (what you see is what you get) developer tool for ASP.NET 1.x.
SharpDevelop
Open source IDE for C# and VB.NET projects. Includes syntax highlighting, code completion (like Intellisense), and code template support.
Utilities that Make Your (Developer) Life Easier
SourceGear Vault
The ultimate source code control solution for .NET. SourceGear is free to use as in single-user mode on your local workstation. Download and install the Server version.
CodeSmith
Template-based code generator that can be used to automatically generate code for anything. It comes with templates for generating things such as stored procedures, collection classes and entity classes. More templates are available in the file share forums.
Unleash It
Utility used to deploy ASP.NET projects. Includes support for zip, FTP, file filters, exclude folders, multiple project profiles, and a Visual Studio .NET 2003 plug-in.
Web Data Administrator
A Web-based manager for SQL Server or MSDE that allows you to create and edit databases, manage users, and edit stored procedures, among other things.
ASP.NET Components
MetaBuilders.com
Andy Smith has created a ton of free ASP.NET controls, some of which are essential for every ASP.NET developer's toolbox. Be sure to check out DefaultButtons, ExpandingPanel, FirstFocus, OneClick, and RowSelectorColumn.
SharpZipLib
From the creators of SharpDevelop comes an open-source zip library for creating and extracting files from Zip, GZip, Tar and BZip2 archives.
QuikSoft FreeSMTP.Net
SMTP component that does just about everything
EmailSpoofer
A control to obfuscate e-mail address links so that spam bots can't harvest them.
eXcentrics World Custom Controls
Matt Hawley, author of Unleash It, has written an impressive suite of custom controls, including a popup calendar, collapsible panel, masked textbox, and FAQ repeater, among others.
FCKeditor
"The text editor for the Internet." Supports IE 5.5+, Firefox 1.0+, Mozilla 1.3+ and Netscape 7+. Be sure to download FCKeditor.NET to get the ASP.NET server control you can use to integrate this WYSIWYG HTML editor with your project.
skmMenu
An open-source ASP.NET menu control created by Scott Mitchell of 4GuysFromRolla fame.
Date Control
A date control that supports multiple languages and date formats. Created by Gavin Lyons at www.i386.com.
HTML Utilities
ieHTTPHeaders
A toolbar for IE that allows you to see the HTTP headers being sent and received.
ToggleBorders
Color codes table, span and div tags so you can visually see how a page is laid out. Use it for debugging or seeing how other web sites are done!
Full Source
Shows the "true source" of whatever Internet Explorer has rendered. Useful for debugging DHTML or XSLT.
Other Tools
FileZilla
Simple and fast FTP client for transferring files. An FTP server is also available.