For posterous.com, we use slicehost VPS, ruby on rails and mysql. Pretty standard setup overall. Separate DB, App and Web server slices, and a separate job server for longer-running postprocesses. S3 for storage and backup.
Slicehost is really popular and with good reason. Linode might turn out to be just as good, but I know the wonderful service I have gotten at Slicehost and how many other sites in the web 2.0 community are there - heck, the Xen blog itself is on Slicehost and not any other Xen provider.