The best replacement for f5 is netscaler.

The best alternatives to f5 are rated as following:
★★★★★  netscaler
★★★★★  f6
★★★★  f100
★★★  cisco
★★★  f55
★★★  ctrl f5
★★  f8
★★  a10
★★  ef5
★★  riverbed
★★  c300
★★  f10
★  fs700
★  f7
★  mig 21
★  kemp
★  mgp10
★  nikon f6
  f4
  garrett at pro
  nginx
  fu
  t38
  ace
  f70
  brocade
  radware
  shift f5
  z5
  ctrl r
  f3 skybox
  canon eos 1v
  foundry
  f3
  citrix
  gary fisher piranha
  ctrl+f5
  rockhopper
  f5
  imperva
  f7 cannondale

Your alternative for f5 is not listed? Suggest!
Speaking about f5, one probably shouldn't forget about vmware view.

Other search terms related to f5 and their ratings are:
★★★★★ vmware view
★★★ splunk
★★ snat
★ fn key
You have in mind a companion for f5 and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of f5 in all years. Click on graph for more.
f5
netscaler
f6