Oregano Server is a Multiuser Server for Flash Clients. It is written in Java. It is useful for client-server applications that require low latency such as real-time chat systems, shared whiteboard applications or multiuser games that would be difficult to develop with traditional HTTP-based solutions

It is free and released as Open Source under the GNU Lesser General Public License (LGPL).

Oregano is on hiatus. It is unlikely that there will be any further releases.
The Client API is available for AS1 and AS2. If you are going to develop in AS3 and/or need features like Video/Audio Streaming and Broadcasting you may want to look at Red5.

Showcase: German TV station WDR used Oregano Server to build their Online Community "Zimmer Frei".
You can register for free at www.zimmerfrei.de/wg/.
Note: This is a German speaking community.