InstaDNS

1-2-3-4.4i.am

A record

2001-db8--f00f.4i.am

AAAA record

err-5.4i.am

Fails with error code 5 (for all values of 5)

c-cloudflare-com.4i.am

CNAME record for cloudflare.com (like so); dashes for dots.

c-toys--r--us-com.4i.am

CNAME record for toys-r-us.com; double dashes for dashes. Yes, this is technically invalid.

3s.1-2-3-4.but-50-pct.2-3-4-5.4i.am

Pick one IP at random (TTL 3s)

3s.1-2-3-4.but-at-1456681051.2-3-4-5.4i.am

Switch IP at a given time (takes a unix timestamp). TTL 3s.

log-foo.4i.am

Log queries to a queue.

dump.4i.am

Return the query itself as a TXT record.