The best replacement for gns3 is dynamips.

The best alternatives to gns3 are rated as following:
★★★★★  dynamips
★★★★★  boson
★★★★  iou
★★★★  packet tracer
★★★  cisco packet tracer
★★  ns3
★  boson netsim
★  opnet
★  dynagen
  netsim

Your alternative for gns3 is not listed? Suggest!
Speaking about gns3, one probably shouldn't forget about vmware.

Other search terms related to gns3 and their ratings are:
★★★★★ vmware
★★★★★ real switches
★★★★★ asa
★★★ juniper
★★★ ccp
★★★ sdm
★★★ junos
★★★ switching
★★ vmware workstation
★★ securecrt
★ putty connection manager
★ asa firewall
★ real switch
★ wireshark
You have in mind a companion for gns3 and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of gns3 in all years. Click on graph for more.
gns3
dynamips
boson