Preserving and documenting Satoshi's initial implementation of Bitcoin software.
bitcoin-preview
consists of the source code that Satoshi shared with few folks in the Metzdowd cryptography mailing list asking them to review his implementation. This is the first known instance of bitcoin implementation being shared.
The code here is what Cryddit shared in this bitcointalk.org thread.
These contain the Bitcoin v0.1.0 ALPHA, Bitcoin v0.1.3 ALPHA, and Bitcoin v0.1.5 ALPHA releases.
Its interesting to dig into things that changed between the preview and v0.1.0 version.