The best replacement for debugger is xdebug.

The best alternatives to debugger are rated as following:
★★★★★  xdebug
★★★★★  emulator
★★★★  windbg
★★★  ollydbg
★★★  pry
★★★  disassembler
★★★  ns-2
★  editor
★  compiler

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

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



Google Trends

Scale is based on the average worldwide traffic of debugger in all years. Click on graph for more.
debugger
xdebug
emulator