The best replacement for trello is asana.

The best alternatives to trello are rated as following:
★★★★★  asana
★★★★  evernote
★★★  jira
★★★  wunderlist
★★★  basecamp
★★  pivotal tracker
★★  todoist
★★  microsoft project
★★  any.do
★  google keep
★  azendoo
★  asana vs
★  fogbugz
★  podio
  leankit
  teambox
  springpad
  kanban

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

Other search terms related to trello and their ratings are:
★★★★★ gtd
★★★★ scrum
★★★ time tracking
You have in mind a companion for trello and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of trello in all years. Click on graph for more.
trello
asana
evernote