I've used onelook.com for a while, which is another aggregator that (for now) seems to have more links than Google Dictionary does.
But Google Dictionary isn't just an aggregator, they provide their own pronunciations for some words (a really important feature IMHO), and a list of synonyms for some words.
I actually hope that onelook links to Google Dictionary, as strange as an aggregator-linking-to-aggregator might be.
My guess is that Google has been working on computational linguistics for such a long time (stemming has been important for search for a while, and Google lately has started throwing in synonyms to the search results) that it's natural for Google to start exposing some of their internal dataset to the world more directly.