Panic – makers of the highly acclaimed FTP client Transmit, and all-round web creation suite Coda. They have some free stuff.
SmileOnMyMac – they have a couple of brilliant utilities, my personal favourite being TextExpander. Also recommend looking into BrowseBack.
Ambrosia – search utility iSeek for quick access to popular general or topical search engines (e.g. Google or IMDB), and deep system software for video and audio capture, and arbitrary iPhone ringtone upload.
Rogue Amoeba – more and similar deep system software, including stuff for audiocasting wirelessly or to the web.
DEVONtechnologies – sell a suite of software that includes a desktop web search engine (it uses your CPU rather than Google’s and gives slightly different results), an application that combines bookmarking, PDF management software, and general file management in one interface (called DEVONthink), and a note taking application. I haven’t been able to make up my mind over whether I’d profit from using any of their software, but their search utility is good when you need more results than Google provides (but it’s not better than Google, it just returns a different set). Also haven’t been able to verify for myself that they have useful artificial intelligence in their software – which they claim. They have freebies.
Unsanity – makers of the well-known ShapeShifter application. Many of their applications are concerned with bringing back useful features from Mac OS 9. There’s some free stuff as well.
Freeverse – quite a ragbag of applications, including several applications for diverse graphics and audio creation tasks, as well as the free “Think!” – a window-shading app. You’ll also find a whole bunch of Mac games on their site which are quite entertaining.