In response to Leonard Will:

The Multi MIMSY system from Willoughby Associates that is being implemented
by the LASSI consortium in the UK has a sophisticated whole/part handling
mechanism that also is applicable to related items (copies of paintings,
members of sets), even if those items are not held by your institution.

You create a record for each of the wholes and parts (collections, groups,
wholes, parts, non-detachable parts, etc.) and then click a button on the
screen that pops up a list of candidate items to be related (you get to pick
from anything in your database or just a subset of relevant items).  You
then click on the item you want to relate and you're prompted to fill in the
relationship.  A reciprocal relationship is automatically filled in for the
related item.

There is no limit on the number of items that may be related this way.
Additionally, the relationships can be hierarchical and have any number of
levels.  For an archive, for example, you can have collection level records
(Abraham Lincoln) that relate to subgroups (correspondence, accounting
records)  that in turn relate to individual items (letters, ledger books).
When you are in a record describing a letter you can just click on a button
and you get a grid showing the related items.  These related items may be
other letters in the same subgroup, the record describing the subgroup if
you have one, or the record describing the collection as a whole.  You can
click on any item in the grid and the system takes you directly to that
item's record.  You can move back and forth between related items and/or up
and down the hierarchy of related items.

Aside from the obvious collection management applications, the function is
specifcially designed to facilitate use of interrelated information on the
Internet or for public learning.   The concept being to provide context, not
just object information.
The system also provides similar interlinking of associated people, places,
events, subjects and themes, and dates.  Not just the recording of such
information within an object record, but rather stand-alone repositories of
information that can then be linked to object records or can be linked
amongst themselves.  For example, you can put in extensive biographical
information on the sitter in a portrait (George Washington) into a people
file and then interrelate this unlimitedly to other people in the same file
(Martha Washington), places in the place file (Valley Forge, Mount Vernon),
events in the event file (Crossing of the Delaware), or subjects in the
subjects file (American Revolution).  Again, the idea is that all of this
information can then be extracted for use in Internet or public learning
situations.