It captures information about Lua errors that occurr during your play session, including the full debug stack. It aims to be invaluable to developers, and a useful tool for players that use beta-quality addons.
BugSack includes a DataBroker plugin that displays the last 5 errors in the tooltip. It can also bring up a frame to display all other errors for the current session, or for any other session. The text of the error can then be copied & pasted to a source outside of WoW (for example, the forums) so that other developers can determine what's at fault.
Errors can also be sent to another player who has BugSack installed, if you know someone that can help you interpret and resolve the errors in your interface.
The addon has a hard dependency on !BugGrabber, which is the addon that actually grabs the bugs. !BugGrabber is intentionally kept small, and it does not use any libraries, to make sure that any errors in libraries do not mess up the system. !BugGrabber is included in the download from this page.
BugSack and !BugGrabber has been hacked on by many people since it was started, including but not limited to: Fritti, Rowne, Ramble, kergoth, Industrial, ckknight, Gamefaq, thomasmo, and damjau.