Hint 1:
Search these forums, a lot of the command line parameters (if not all - except for the very new ones which you can read about in the changelogs) are already explained somewhere in one way or the other....
Hint 2:
Read changelogs here or here.
Hint 3: Use a tool which can read the contents of a file.
In other words, even with notepad you could find out most (if not all) available command line parameters. This isn't so convenient though, but a tool like
PE Explorer (used by people for resource hacking as an alternative to the tool
ResHacker) can list you all the strings within an exe. In that way, it is only a matter of searching for stuff beginning with "/" or "-" (in case of Messenger Plus! even " /" in some cases (thus with a space in front)).
note: of course an official list like the
registry settings list would be nice. But many of those command line parameters are actually of no use for the end-user anyways, or even not for the advanced user (refering for example to the explaination of the workings of the old
/WinStart parameter)
From what I remember and from what I can quickly find in my old notes:
MsgPlus.exe (Plus!3):
/ExtractSponsor
/SndPack:
/Log:
/AutoStart
/WinStart
/Remove
LogViewer.exe (Plus!4 and up):
/help
-help
/ViewLog=
/Encrypt=
/Dycrypt=
/KeySize=
/Dest=
/Pwd=
/ChangePwd=
/NewPwd=
/Overwrite
/Partial
/RootDir=
/UserID=
/Hint=
/AllowDuplicate
MPTools.exe (Plus!4 and up):
/PlusSkin=
/RestartMessenger
/ImportScript=
/ImportPrefs=
/ImportSoundPack=
/ImportSkin=
/UIB=
/Def=
/Style=
/Dest=
/Src=
MsgPlus Setup (Plus!3):
/SilentInstall
/SilentInstallNosponsor
/NoAdmin
/ExtractSponsor
/ppath:
MsgPlus Setup (Plus!4 and up):
/help
-help
/Destination=
/Silent
/AutoUpdate
Messenger (Plus!4.5):
/PlusEnabled=
/PlusSkin=
Notes:
* Some links might be invalidated by now or might point to the beta testing forums and can only be accessed by beta testers.
*
Some parameters might have been removed or only available for specific versions
* Because I quickly compiled this list I can't garantuee that this list is complete.
* Some parameters should be handled with great care! Remember that almost all parameters are meant to be used internally by Plus! and not by the user, except for a few.
/ 
