WHO ARE YOUR AUDIENCES?
Goodness knows there's such a things as over-gadgetizing a site.  The second key is to make your site optimally useful for your major user groups.  (Web work is grounded in philosophical utilitarianism.)  You won't know at first launch what this will mean design-wise.  Striking the right balance of features will mean responding to user behavior.

International Children's Library
Graphical organization: clear conduit to online book collection.
Enhancements: Popular searches, News, Book of the Day widget, Featured Books, Book summaries, multiple translations, user reviews, customized searches (by unusual criteria like cover color, award winners, "find similar," animal characters, "true or makebelieve," feelings)

Curating the City
prominent interactive Memory Board

On This Day
interactive "witness" to history memory board
multimedia sidebar
helpful notice: "This site is archived and is no longer updated."

Internet Archive
news, RSS, forum, user reviews, education section

Hymnwiki or International Music Score Library Project
Hymnwiki: interactive by its wiki nature
IMSLP: enhanced wiki (browse, recent, news, graphical cues, featured, how to, community page, forum)

David Rumsey Map Collection
Blog, multimedia tour, Google Earth & GIS enhancements

NYPL Digital Library
daily gallery, news, graphical cues

WHAT ELSE DO YOU KNOW?
Take advantage of the medium, and be expansive with what you offer users.  If well-organized, the extra data you can provide may be the key that makes your collection work, or it may be a part of your collection's long tail.

Biodiversity Heritage Library
"Now Online" statistics tell users how many titles, volumes & pages are to be found behind the site walls.
A tag cloud gives direction to browsing.

American Journeys
thorough teachers' section, including guide to handling sensitive content

Project Gutenberg
top lists, news

Mutopia
Browse by instrument or style

Ackland
Five Faiths Project educational component (okay, I just included this because I worked on it)