I like Panic's applications, which show a great dedication to the Mac OS X look-and-feel and usability. Have bought Transmit and will most likely do so with Unison after my still running trial period runs out.

Few issues: Occasionally crashing, font appearances is different to e.g. Apple's Mail.app for my favorite fonts (Myriad Pro, an .otf font, for message lists, or Andale Mono for message text... this can be observed for other fonts as well)