The best replacement for chatter is yammer.

The best alternatives to chatter are rated as following:
★★★★★  yammer
★★★★  prat
★★  hipchat
★  socialcast
★  newsgator
★  jive
★  tibbr
  gossip
  snapper

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

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



Google Trends

Scale is based on the average worldwide traffic of chatter in all years. Click on graph for more.
chatter
yammer
prat