I use Tucows OpenSRS (I'm a reseller, but otherwise, just find someone reasonable who is a reseller -- it used to be free to join, then $100, now I think $250).

Basically direct access to both a website and an API for registry functions. If you register enough domain names internally, it's totally worth it. I've never had a problem with them.

They started out as a linux-friendly ISP in Canada, I believe, and have become basically a reasonable registrar -- one of the first group of 34 independent registrars approved by ICANN, and I think the first to really work with resellers.

Hover, which Marco suggests, seems to be a retail part of Tucows.