<p>I was currently looking into memcached as way to coordinate a group of server, but came across <a href="http://hadoop.apache.org/zookeeper/" rel="noreferrer">Apache's ZooKeeper</a> along the way. It looks interesting, and Yahoo uses it, so it shouldn't be bad, but I'd never heard of it before, so I'm kind of skeptical. Has anyone else given it a try? Any comments or ideas?</p>

ZooKeeper and Memcached have different purposes. You can use memcached to do server coordination, but you'll have to do most of this work yourself. Memcached only allows coordination in that it caches common data lookups to be used by multiple clients. From reading ZooKeeper's documentation, it has a much broader focus than this. ZooKeeper seems to provide support for server clustering, which isn't the same as the cache clustering memcached provides.
Have a look at Brad Fitzpatrick's Linux Journal
article
on memcached to get a better idea what I mean.