PART files are used by several file transfer programs, including Go!Zilla, eMule, and the discontinued eDonkey program.
If you are using Skype on a Mac and a PART file keeps reappearing, move the file to the trash and make sure it's the only file in the trash. Then, right-click the "Trash" icon, hold the "Command" key, select
, and verify that the trash is empty. Now, when you reopen Skype, the PART file should no longer reappear.
NOTE: To resume a PART file download with FlashGet, you may need to rename the extension to .JC.