The new version of Keynote (found in iWork '06) adds some great features, including some new transitions between slides and builds for the way content enters a slide.
Curiously, this new version seems to omit the Twirl, Motion Dissolve, Grid, Falling Tiles, Drop, and Burn transitions, and the Drop and Twirl build effects. However, you can easily get them back.
In Keynote 3, open the preferences and select the General tab (if it's not already selected), and you'll see a box at the bottom that says:
Animations: include obsolete animation choices.
Put a check in the box next to this, and the missing transitions and build effects return. Why they consider them to be "obsolete," I have no clue!
[robg adds: I noticed the missing effects when I ran through iWork during Expo, but I didn't notice the checkbox, so I felt it worth sharing. It seems quite odd to include the animations themselves, but then hide their availability behind a checkbox marked "obsolete."]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060118144601537