WeiDU can be downloaded from this post here at ppg: http://forums.pocketplane.net/index.php/topic,21324.msg279310.html#msg279310
WeiDU stands for Weimer Distribution Utility. Wes Weimer was the original developer and maintainer of the utility. Since version 185 it has been in the capable hands of the Bigg to maintain. Weidu can be used for mod distribution as well as file editing of Infinity engine games. It will take a bit of study and practice to properly understand what can and can't be done. Weidu operates by using a variety of command line functions. If weidu.exe is renamed as setup-somemod.exe (where somemod is the name of the mod) it will automatically run a tp2 file named somemod.tp2 or setup-somemod.tp2. The main function of weidu these days is using it as setup-somemod.exe to distribute mods, and to modify the users files at mod installation time rather than copying over files that may wipe out changes made by previously installed mods. Please note that just because a mod is distributed or made with weidu, it doesn't mean that it will play nicely with all other mods. Some mods can change the same file in different ways bringing with them the possibility of conceptual incompatibility or even technical incompatibility.
Conceptual incompatibility: where mod A changes a file and mod B changes the same value in the same file to be something else. In this case, last mod installed wins.
Technical incompatibility: where mod A completely removes something from a file (an extra sword from a creature as an example) and mod B wants to move the sword from the inventory slot to a weapon slot. In this case, mod B would produce an error unless the mod author put in enough checks to ensure that the sword was indeed still there.
Near Infinity can be downloaded from the link in this post at G3: http://forums.gibberlings3.net/index.php?showtopic=12896&view=findpost&p=112339
Near Infinity is a GUI based editor for all the Infinity Engine games. There is some support for Neverwinter Nights as well. It requires the use of a certain version of Java or higher to work. Which based on my experience requires Windows XP or higher. There is an older version of NI which may run on Windows 98.
Near Infinity displays the exact offset from the beginning of the file for each value.
DLTCEP can be downloaded from this post at G3: http://forums.gibberlings3.net/index.php?showtopic=11824&view=findpost&p=100771
DLTCEP is a GUI based editor. It does not show any offsets unless the entry is an unknown. Many people find DLTCEP to be easier to use than NI. Both are good programs to use when you want to modify an existing file and save it as a new file to be distributed with a mod. When making a new creature, item, spell, area, or what not it is always best to use a pre-existing file of that type as a base. This is mainly from personal experience. There are unknown values or other known entries that need to be set a certain way for the file to properly work within the game. When creating a new file, if you don't start with an existing file as the base, you may end up having problems trying to get it working within the game.
IESDP doesn't need to be downloaded (although you can have a local copy if you want). It can be found here: http://iesdp.gibberlings3.net/
IESDP is a good source of information when you want to find out about the various types of files in the game. It is a good companion to have when creating tp2 components which will edit various infinity engine game files.
I hope that helps. There are more programs out there, but I have not used them so I can't really talk about them. Also search the various Infinity Engine modding forums. Most of them have tutorial sections and/or download links to a variety of tutorials and tools. If you don't know many of the modding sites, here are a few...
Black Wyrm Lair at http://www.blackwyrmlair.net/
Pocket Plane at http://pocketplane.net/mambo/
<-- you at least know this one...
Gibberlings 3 at http://gibberlings3.net
Chosen Of Mystra at http://chosenofmystra.net/
Spellhold Studios at http://www.shsforums.net/