What are people using for payment processing?

I have used Authorize.net and similar providers in Europe several times, and the RYO approach is pretty simple, enough so that I'd recommend it over a separate solution unless you have well tested that solution and your needs are sufficiently complex. The caveat is that I have only done this with WebObjects, not Rails, but I don't anticipate any problems in a Rails implementation.

Paul

I've used Authorize.net and Verisgn's Payflow Pro both with Rails.
Authorize.net is easier to use but Verisign works.

I'm probably going to use TrustCommerce on my next project.