The Mojo SDK includes all the tools that you will need to program apps... except a text editor. While you can certainly use any text or programmer's editor to create apps there are a few IDEs that wrap Palm's command line tools.
Integrated Development Environments
Palm's own web-based IDE is Ares. Ares is a very capable development environment.
Komodo with webOS Add-on
Komodo Edit (free) or Komodo IDE (commercial) paired with the Komodo webOS Add-on from Templarian provides a very powerful, cross-platform environment for developing webOS apps. In addition to the standard features included with the Palm plugin for Eclipse the Komodo IDE includes logging functions, an appinfo.json editor, the ability to execute arbitrary commands on the emulator, TODO tracking and more.
There is a Bundle for TextMate which allows you to use main features of the Palm SDK and snippets for Mojo from within TextMate, to ease and fasten the App development. More information can be found here: 
The current version of the Bundle can be downloaded from GitHub: 
Jetbrains, the makers of Intellij and other IDEs, have created a new web development IDE. To get intellisense for Mojo properties, add the Framework directory( i.e. /opt/PalmSDK/Current/share/refcode/framework/330) as a content root. See  for more info on Webstorm.
- There is a plugin for Coda, a Mac OS editor: Coda Plugin
SimFinger Screencasting tool
SimFinger is a screencasting tool for the Palm Pre and Palm Pixi emulators. See http://erickreutz.github.com/SimFinger/ for more info.