If you haven't already done so, you might want to read the intro to hidden preferences. Or if you're in a hurry, just enter the commands below in the terminal, and then restart the Dock to see your changes.
In this category I like to start off with getting rid of the stupid 3D:
defaults write com.apple.dock no-glass 1
and then (as you'll have to do after every defaults command on this page, killall Dock. Of course, you can change several settings before restarting the Dock if you want.
defaults write com.apple.dock showhidden 1
To cause the dock icon of hidden applications to be dimmed.
defaults write com.apple.dock mineffect suck
In case the "genie effect" isn't flashy enough for you. Especially nice if you hold down shift while minimizing.
defaults write com.apple.dock workspaces-auto-swoosh 0
To avoid being jerked around when you switch apps. In 10.6, the default behavior is much better, but in 10.5, I couldn't stand using spaces without this, because you'd occasionally get whisked away even when there was a window for that app in your current space.
There's only one I'm putting here so far, but it's new in the 10.6 Dock, and I haven't seen it listed elsewhere yet.
defaults write com.apple.dock use-new-list-stack -bool YES
This makes the list display for stacks behave like the grid view, except with items arranged in a single vertical column with 32px icons and labels to the right. I don't like it personally, 'cause I prefer the regular list view to grid view anyway, but maybe some of you will.