The first wave of peer-to-peer systems included several systems (The Eternity Service [1], Archival Intermemory [4], Free Net [5], and Free Haven [7]) designed to provide uncensorable storage: documents once deposited in these systems cannot be altered or destroyed by attackers, even national governments. Like ours, these systems use cryptography and redundancy (Archival Intermemory uses erasure-correcting codes) to protect data. It was soon suggested that one of these systems would make a good base on which to build a backup system.
Unfortunately, however, these systems do not have working defenses against free riders when used in this way: Freenet keeps only the most popularly requested documents, Archival Intermemory simply assumes cooperation (it is intended for research libraries, which are believed to have substantially similar interests), and the Eternity Service paper only suggests a possible line of direction for a defense.
The Free Haven project has proposed an elaborate scheme based on
trading storage, where nodes are allowed to insert only as many shares (the unit of trade in Free Haven) as they hold for others.
However, unlike in our system, these shares are constantly traded
between nodes, leading to effectively non-symmetric partnerships. This
prevents a node 
from directly punishing a node 
that
drops one of 's shares by dropping one of 's shares that
holds in return. Instead an elaborate reputation system is
needed to punish nodes that are complained against too many times
because they drop data. Unfortunately, building a working reputation
system is very hard and it is far from clear if their system
works [8].
More recent peer-to-peer storage systems include PAST [10,14] and OceanStore [12,15]. These systems do not attempt to provide uncensorability, and are thus simpler than the previous systems.
PAST relies on trusted third parties and smartcards to broker requests between clients so that clients cannot use more remote storage than they are providing locally. Unfortunately, insufficient details are provided about PAST's defenses to properly evaluate PAST's resistance to free-rider attacks--e.g., nodes are supposed to be randomly audited, but no details of how or with what consequences are provided. PAST appears to encrypt data before replicating, which may allow a free-rider attack where malicious nodes obtain data from the redundant data that their peers store, rather than storing it themselves (see Section 3.1).
OceanStore is a federated system where utility companies pool their resources to provide storage to users. Each user contracts with a single company, the responsible party, to receive storage for a fee. That company then exchanges storage with the other companies for greater reliability and geographic range. Because of the companies' large size and deep pockets, legal contracts and enforcement can be used to punish companies that do not keep their end of the bargain, based on planned billing and auditing systems. OceanStore, because of its need to support concurrent updates, is very complicated (e.g., it uses Byzantine agreement) and requires a great deal of central resources, making it likely more expensive than our scheme if only simple backup service is needed.
The only other peer-to-peer system we know of whose primary purpose is backup service is Pastiche [6]. Like in our scheme, Pastiche nodes form partnerships with other nodes. However, rather than using erasure-correcting codes, each Pastiche node stores a copy of all of its data on each of its partners. By sacrificing a fair amount of privacy (observers can tell if a node's filesystem includes a given large byte sequence), this backup data can be greatly compressed: by choosing partners with similar software installations, most files being backed up will be already present on the partner, thus requiring no extra storage. No data is available yet about whether this savings compensates for the greater overhead of using full replication rather than erasure-correcting codes in practice. Pastiche has not yet adopted defenses against free-rider attacks; the authors merely sketch, in a paragraph each, three approaches that they are considering.