uxRefresh has collected a handful of examples of A-Z indexes of titles on large sites. Also useful might be a look at the article I wrote about KWIC/KWOC/A-Z Site Indexes work I did while at Bell Labs, and for a deeper dive, look at the list of links of indexer-designed examples on the Montague Institute's site. Many of the latter are still valid, but there's some link rot there too. James Kalbach also recently wrote on A-Z indexes, and in particular demonstrated the use of indexes in search auto-completion using Elsevier bibliographic database EMBASE.
Exhaustive A-Z indexes of the variety that indexers produce with relationships among terms are rarely seen on larger sites. This is probably for good reason--search engines are often as effective in most cases and indexes requiring human, subject matter expert involvement are expensive. The kinds of places where these A-Z indexes are to be found are largely in bodies of content with some sort of specialized need, e.g. research databases. It's important to keep in mind, when comparing the above, that there is a difference between simply providing an index of titles of content, and providing an index of terms found in the content. The latter, in my opinion, is where the display of relationships and hierarchy can be most relevant and useful.