The best option for Google Wave replacement really depends on how you used it. I think that was the problem… there was so much that could be done with it that it failed to grab a significant niche. That and the UI difficulties that led to overwhelm.
If you just want the real time document collaboration, Google Docs works pretty well for that. It’s probably not the best for long, multi-faceted discussions, but it could be co-opted.
If you just want collaborative discussions, you could try Google Shared Spaces. It’s pretty easy to set up and use, but seems more disposable in nature. Here’s an overview: http://www.40tech.com/2010/12/22/collaborate-on-the-fly-with-google-shared-spaces/