The best replacement for sametime is same time.

The best alternatives to sametime are rated as following:
★★★★★  same time
★★★★★  lync
★  communicator
  lotus notes

Your alternative for sametime is not listed? Suggest!
Speaking about sametime, one probably shouldn't forget about lotus notes.

Other search terms related to sametime and their ratings are:
★★★★★ lotus notes
★★★★ pidgin
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sametime
same time
lync