- Text Editor (Emacs has a good scala mode, TextMate has a good syntax highlighter too)
- Intelli-j (So I have heard)
I think for all the IDEs the code completion is not 100% yet. Scala 2.8 has added several hooks into the compiler for IDEs so the support will be must better.
Scala 2.8 is scheduled for release end of 2009 beginning of 2010.
General -- http://scala.sygneca.com/tools/start
Emacs -- http://scala.sygneca.com/tools/emacs
YAS+Emacs -- http://www.scala-lang.org/node/354
Netbeans -- http://wiki.netbeans.org/Scala#section-Scala-GetStarted