The best replacement for nessus is retina.

The best alternatives to nessus are rated as following:
★★★★★  retina
★★★★★  openvas
★★★★  nmap
★★★★  nexpose
★★★★  metasploit
★★★  qualys
★★  mbsa
★★  ncircle
★★  saint
★★  acunetix
★  core impact
★  iss
★  snort
★  webinspect
★  gfi languard
★  foundstone
★  mcafee vulnerability manager
  rapid7
  nikto
  zenmap
  appscan
  languard

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

Other search terms related to nessus and their ratings are:
★★★★★ metasploit
★★ nmap
★★ nikto
You have in mind a companion for nessus and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of nessus in all years. Click on graph for more.
nessus
retina
openvas