Have a look at ShipWire.com. They are great at B2C tools for shipping individual items to end customers and they provide integration with largest number of eCommerce platforms. They are not very good at B2B though but depending on your requirements that may be a good fit for B2B too.
Another company is Smart Warehousing. They are good at B2B and may fit nicely for B2C as well. The challenge is that they don't provide pre-packaged integration but they expose their SOAP API instead. If you can develop your own integration then they may be a good fit too.
I ended up using ShipWire for B2C and SmartWarehousing for B2B. This is not ideal but works for us at the moment.