ZoneEdit.com has led a long history in free DNS hosting service. It is reliable, feature rich and free for the first 5 zones you hosted with them. They do quite a bit more than simply returning you the DNS records. Here’s a description from their FAQ page:

What services does ZoneEdit provide?

ZoneEdit hosts reliable DNS servers, e-mail servers for email forwarding, and web servers for URL forwarding and parked domains. ZoneEdit provides a convenient single-location, integrated, web-based domain manager for configuring all of the services provided.

I have been using ZoneEdit since late 2002, and currently have 2 domains delegated to their servers. In one stage (~2 years ago) the web front-end was giving me hard time, but the DNS itself has been flawless. If you like free services, has less than 5 domains to manage, want features like backup MX and automatic failover service — then ZoneEdit might be the provider for you.