~10mb

• [6mb] Micro Core
• [10mb] Tiny Core

~50mb

• [50mb] Plop Linux — officially supports PXE+NFS booting
• Trinity Rescue Kit — officially supports PXE+NFS booting
• [50mb] SLAX Core (use 'build' to get an ISO for this)
• [50mb] DamnSmallLinux
• RIP (Recovery Is Possible) — officially supports PXE+massive-initrd booting
• Knoppix — see its Terminal Server