The Salitronic Gerber Analyzer is a free online Gerber viewer and PCB analysis tool. It needs JavaScript and runs entirely in your browser, so your files are never uploaded.
It reads Gerber RS-274X and X2, Excellon drill, ODB++ and IPC-2581 files (ZIP, RAR, 7z and TAR archives too) and includes a 2D and 3D viewer, stackup editor, net tracing, design rule check, Gerber diff, thermal simulation, impedance probe, EMI and crosstalk analysis, power integrity (IR drop), antenna analysis, a pick and place and BOM viewer, and a PDF fabrication spec sheet.
Read more about the Gerber Analyzer or ask for an independent PCB design review.