Probably the easiest to use, and officially supported, is Eclipse.
The Eclipse Android Developer Tools (ADT) plugin allows for basic project creation and management, as well as step debugging and ability to easily export your app for deployment/publication.
http://developer.android.com/tools/index.html
Google now distributes an ADT Bundle, which includes Eclipse with the ADT plugin, SDK and platform tools, and latest Android platform and system image for the emulator.
http://developer.android.com/sdk/index.html
The Android SDK Tools alone can still be downloaded if you prefer not to have the entire bundle, or if you already have Eclipse installed and just want to add the ADT plugin and SDK/platform tools, or if you prefer to use another IDE.
http://developer.android.com/sdk/installing/index.html