Diskus is a high-level hard drive diagnostic and limited repair tool. It checks the disk geometry by writing numbered IDs into the sectors of the hard drive. Later one can verify that these sector-IDs on the disk are still correct, thereby detecting data which was not correctly written. There is an option to "freshen" the data on your harddrive, which will read and rewrite the sectors. With modern drives this will re-map defective sectors.
md5chk is a tool for creating MD5 sums. It was designed so that its output would be easy to use with command line shells. It guarantees that the MD5 sum is always "clean" of characters that would be misinterpreted by the shell and that special characters in the file names are always properly escaped.
mod_gfx is a configurable Apache module that uses libgd to perform image operations on the fly. Some of the supported image operations are: resize, resample, watermark, and crop. A couple of its special features are origin selection via URI regex and round-robin over a list of hosts.