This step-by-step procedure will show you how to make a DMG package from the GNS3 source code. I’ve built the package on Mac OS X Snow Leopard and used Py2app to make a Mac OS X application. Also, have a look at this article that really helped me.
This procedure worked for me, please let me know if you encounter any issue and I’ll update it. Also, please tell me if you can make it work on Mac OS X Leopard (10.5.x).
Here are instructions to run GNS3 from the sources (not the standalone compiled DMG version) on your favorite operating system. You will have to install or compile dependencies like Qt, Sip and PyQt. This will allow you to use the latest development version of GNS3 that you can download from http://code.gns3.net.
In this tutorial we’ll use MRTG program to get traffic statistics from emulated routers in GNS3.
In this article, I will show you how to emulate Cisco ASA using Qemu. Once again, please note that ASA is not provided and will not be. So please don’t ask. Also be aware that ASA does not 100% work in Qemu but that’s enough to play with it.
This Howto is still a draft and has been tested only on Linux.
This article explain you how to emulate Juniper JunOS on a PC using Qemu. This is an updated and enhanced version of excellent howtos from Juniper Clue and Internetwork Pro as well as Himawan Nugroho’s blog. I mainly focused on Qemu, so if you wish to install JunOS on a real PC or using VMware, please have a look at the Juniper Clue article for more information (and of course Google).
So what’s new you would say? First, I chose to use the latest version of Qemu: the 0.11.0 which supports the Intel e1000 network card emulation since version 0.10.0. and includes several fixes for it. I have modified and adapted the old patch for Qemu 0.11.0, it includes the UDP tunnel (connection to Dynamips/GNS3), PCAP and LCAP support. Also, the patch allows multicast traffic with the e1000, i82557b and i82559er Qemu emulated network cards. Moreover, this article show how to emulate JunOS on multiple operating systems: Mac OS X, Windows XP and Linux Ubuntu 9.04 without using an untrustworthy obscure binary downloaded from a forum you can’t even read the language 
» Read more: Olive reloaded or how to emulate Juniper routers