Malk's question was about encrypted p2p which might be strictly interpreted to include only virtual private networks. Assuming there is also interest in open anonymous networks:
MUTE seems to enjoy fairly good popularity. According to project's Sourceforge homepage some 400 people download it daily, and it has been downloaded already 800,000 times over the past 3 years. (Despite of this the poor developer has managed to collect only $43.98 in donations to support his work.
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