While gif based banners are still more prevalent on the web, flash based banners are becoming more popular. Flash or .Swf banners allow animation and interactivity that animated gif banners simply cannot match. They also offer dynamic movement with lean file sizes. Here are a few examples.
Roll over to see at full size and click to view the animations.
Using a semi transparent mask and script to smoothly animate changing text as the plane flies over would have been impossible using animated gif format.
As the bulbs flash the text is updated in this red carpet styleanimation.
Animated text(using swift) makes this an eye catching banner.
A long animation that again owes its effects to the unique abilities of flash.
An irreverent use of toilet humor avoids the 'bog' standard look through adobe flash.