GNS3 is one of the Network Simulators, that allows you to use real router images and practice on real router CLI. Generally it is used for Cisco devices with IOS images. But you can also add Juniper JUNOS and Nokia TiMOS images, and you can use this devices on your simulated network.
GNS3 is especially an important network simulator for you during your certification studies. For Cisco CCNA, CCNP, CCIE or For Juniper JNCIA, JNCIP, JNCIE or for Nokia certifications like NRS I, NRS II and SRA, you can use GNS3 for hand on experience. With this Network Simulation Pragram, you can practice on various vendor products and increase your experience on various CLIs. You can download GNS3, here www.gns3.com