The best replacement for ustream is youtube.

The best alternatives to ustream are rated as following:
★★★★★  youtube
★★★  twitch
★★★  live stream
★★  qik
★★  justin.tv
★★  justin tv
★★  blogtv
★★  justin
★★  twitch tv
★  google hangout
★  stickam
★  livestream
★  watershed
  twitcam
  stageit
  jtv
  veetle

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

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



Google Trends

Scale is based on the average worldwide traffic of ustream in all years. Click on graph for more.
ustream
youtube
twitch