I currently use stamps.com. Pricey for low volume mailing though they add enough to make me stay as a customer. However, their software and service is pretty ratty - server outages (4 hours a few days ago), chokes on some European language characters and weird random errors with inscrutable explanations that crop up from time to time. And trying to get a customer service person on the phone is an exercise in futility. Plus they have a very limited range of supported label types. I took a look at using the USPS stuff and found that to be really bad plus I always seem to get a slightly higher postage cost with them.
What are people using? Cheaper would be great but what I really want is reliability and software that looks like it was actually written in this century.