Extraction and decompression software for your mods  

If you can't unZip your new downloaded mod - this is the place to look!

Stuffit
Free download: http://www.freedownloadscenter.com/Utilities/Compression_and_Zip_File_Utilities/Aladdin_StuffIt_Expander_for_Windows.html
 Stuffit handels zip, sit, arc, arj, Bzip, gzip, Lha, sitx, mime, tar, cab, lzh and rar-archives.
Stuffit provides support for most compressed and encoded files, as well as an easy to use interface that integrates with Windows Explorer. Free trial download. Official site: http://www.stuffit.com/win/index.html

7-zip
Free download: http://www.7-zip.org/  
Highest compression ratio in new 7z format with LZMA compression.
7-Zip is free software distributed under the GNU LGPL.
7-Zip Supported formats: 7z, ZIP, CAB, RAR, ARJ, GZIP, BZIP2, TAR, CPIO, RPM and DEB  
WinRar
Official site: http://www.rarlab.com/  
WinRar handels rar-, zip-, cab-, arj-, Lzh-, ace-, tar-, Gzip-, uue-, bz2-, jar- and iso-archives.
WinRar is easy to use and integrates with Windows Explorer. Free trial download. 
WinAce
Free download: http://www.winace.com/
WinAce compression of the following formats: ACE, ZIP, LHA, MS-CAB, JAVA JAR 
decompression of: ACE, ZIP, LHA, MS-CAB, RAR, ARC, ARJ, GZip, TAR, ZOO, JAR

ZipGenius
Free download: http://www.zipgenius.it/index_eng.htm
ZipGenius is free and easy to use, plus it supports more than 20 formats of compressed archives,
inlcuded RAR, ARJ, ACE, CAB, SQX and the excellent 7-zip.  

Powerachiver
Free download: http://www.powerarchiver.com/  
Powerachiver provides support for most compressed and encoded files, as well as access to many powerful features and tools through an easy to use interface that seamlessly integrates with Windows Explorer.
PowerArchiver is a truly global archive utility, fully localized in over 10 languages,
including French, German, Italian, Czech, Finnish and many others.  

WinZip
Official site: http://www.winzip.com/
WinZip The most popular compression utility for Windows, with more than 130 million downloads
Quickly and easily zip and unzip files and folders.
Save storage space, dramatically reduce e-mail transmission time, and efficiently archive documents.
Easy-to-use AES encryption to protect your sensitive data.
Flexible user interface: the intuitive WinZip Wizard for first-time users and the award-winning WinZip Classic interface for power users. Free trial download.  

A good place to start for beginners is
Starting out with mods on Morrowind Mythic Mods.

If you have trouble to add a new mod to your game - always read the ReadMe-file which came with the mod.
 Grumpy's Mod Installation Tutorial can also be a good help.