Ask HN: Can you recommend me a modern US stockbroker?

I'm looking for recommendations and advice regarding choosing an online stockbroker, and figured HN would be a great place to ask. Since I last traded anything, several years have passed and I've moved from the UK to the US. In the UK, I had an account with TD Waterhouse - they had a passable web interface (for the time) but fairly high commission. I'll be making medium-to-long-term investments.

Since there's probably a fairly good baseline of trustworthy brokers with reasonable fees, here are some specifics I'm interested in:

* Great customer service, appropriate to my intermediate-level experience trading and limited familiarity with US tax laws, etc.

* Great web & mobile user interface - I'm a web engineer; it hurts to be forced to use something shoddy.

* Availability of an API - not a killer by any means, but I'd love to be able to write my own user interface. Not for automated trading; just for tinker value.

Any suggestions will be much appreciated!