Q:

How do I catalog files on several external hard drives that I want to store off-line? 

I have quite a few external hard drives that I want to store off-line, and possibly off-site. 
The contents is not saved in a structured manner, however; basically  there's lots of project data for lots of projects. 
Is there any software for Windows XP that will catalog all the hard drives, so I can search the catalog, find the files, and get the ID of the disk that has the content? 

A:

My favorite is by far Cathy!  It's very fast!  works superbly, creates small compact catalogs.   Its only 59KB in size.  I bet you can't find a tinier app for this purpose anywhere else! 
See here for other alternatives.  
My ideal solution is something like Cathy with these features as well:

Auto snapshot Drives on a scheduled period, storing incremental images. ( You can sort of do this using a task scheduler and git or mercurial I suppose, but haven't tried it yet)
Have a compare option as well. ( See FilePro which has this option)
Then sync my various PCs and hard drives via dropbox/others

And voila!  I have instant search access to all my Disks, plus a backup archive to see my list of files in case of disaster. 
I just found ScanFS today, but it had some errors while scanning a catalog, and not as Hard Drive centric as Cathy.  
Oh that reminds me, I forgot my most often used one that has now been unsupported since 2011 http://locate32.cogit.net/
That can also be used as a cataloger though it requires more effort in adding databases.  Great for searching but poor for browsing. 
So my final recommendation is still cathy.  Damn I have so many great ideas for Cathy, but  the author is unwilling to release the source (i asked). 

Some more research links:

http://www.portablefreeware.com/index.php?id=1716
http://www.indexyourfiles.com/
http://www.softprime.com/

A:

See the freeware Gentibus CD.  
It works well for all file types and has a good and quick search function (that you can see in the image below):

