The best replacement for sametime is same time.

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

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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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same time