I just found this thread, and I'd like to comment on it, even thought it's quite old:
Acquisition has a proürietary GUI but uses the GPL licenses LimeWire Core.
It might be that the developer found a way around the GPL, but it's a narrow issue.
About Alternatives:
Phex is a real alternative if you look for a less resource hungry replacement which offers advanced features like private networks and decentral content distribution.
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Phex - Home
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Phex (Cross-platform) - Gnutella Forums