I wrote one called TipOff
Since it's a shell extension it only works in 32 bit windows. The way around it may be to use a 32 bit File Manager such as Free Commander.
It uses the Windows ToolTip shell extension. For this reason it can't be set to work with every file. It has to be registered per file type. It supports these file types:
.7z .arc, .arj, .cab, .exe, .gif, .gz, .jpg, .lzh,
.msi, .rar, .rpm, .tar, .tgz, .txt, .Z, and .zip
The idea was to add comments to downloads.