For me, Stripe was a revelation because they eliminated fixed monthly costs. When I ran a side project that took credit cards, I found the cheapest option to be a $20/month merchant account + $20/month to Spreedly. Others had a few months free trials, etc. but ended up being more expensive over the first year.

Stripe, while being more expensive than these options per transaction, actually cost me a lot less to start out. While $40/month doesn't seem like a lot, for a hobby project that likely won't make that much money in the first months to a year, it's huge.