I rewrote aalis lgp tools to support repacking, useful for mods etc. so all files don't need to be dumped to disk just to overwrite a few.
The main archive contains the source, feel free to use the lgp interface in any projects.
ulgp v1.2ulgp v1.2.1 (forces lowercase files internally)
ulgp v1.3.2 (no memory mapping, use if you get memory errors)
App tl;dr:
Extract magic.lgp to a folder named "magic"
ulgp -x magic.lgp
Create magic.lgp from a folder named "magic"
ulgp -c magic.lgp
Overwrite files in magic.lgp from those in a folder named "magic"
ulgp -r magic.lgp
GUI should be self-explanatory, the two versions are to match the xp and vista/7/8 visual styles
Run install.bat to associate .lgp files with ulgp, once installed simply double click on an .lgp file to extract to the default folder. Hold shift and right click on any folder to create / add to an .lgp file using the files inside.
Read the readme for advanced usage!, e.g. if you need to distribute mods etc.
Major thanks to Aali for the original lgp code.