I always enjoy Petrilli's blog entries (both technical and political), and his discussion today on augmenting our brain's memory capacity with indexed and cross referenced programming manuals, PDF books, etc. seems right on.
I have been more frequently buying PDF books: not as nice to read, but in the future it is great to have indexed and online. Because I mostly use a MacBook with open connections to my customers' servers for most of my work, my MacBook is the home for my "second brain". I have experimented with doing my own indexing and document clustering (naive O(N^2), hierarchical, and k-means), using Spotlight, and using Google Desktop for the Mac. I have settled on using Spotlight, but using the text, HTML, and PDF's in my "second brain" for my own research programming for IR, etc.