Jeff of Jeff's Gameblog showed his best of the best.
My Best of the Best is my entire Harn collection which I don't use for playing in Harn or Harnmaster. It serves as the best damn game reference for all things medieval.
What I use right not in play are these.
Of course the Swords & Wizardry Core Book. I really should reformat it for digest size and get it printed for my own use.
In the lower left is my Majestic Wilderlands, the lower right the calender system I use. Inside is where I keep track of time.
In the upper left is Chgowiz's S&W Reference sheet followed by my Price list for the Majestic Wilderlands. Really need to get the Magic item pricing down pat right now we are using my old GURPS magic pricing as guideline.
In the center is the Paper Mache book where I keep all my digest sized stuff. I have separate book where I keep my original D&D stuff.
In the upper right are the three harn charts I use the most. The travel times, on it's back is the infamous Harn weather tables. Then a double sided Harn Encounter chart. I use this in conjunction with the charts inside the S&W Reference especially for fantastic creatures.
Along with this I would normally have a campaign binder but I don't have one, yet, for the Viridistan area. Slowly building it up as the players explore.