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Textmate is, hands-down, the best text editor I have found for the Mac. Many go as far as to call it an IDE (Integrated Development Environment) as it is feature-packed, and extensible.
Textmate handles many types of text files, with many languages supported. If your favourite language seems to be missing, there will almost certainly be a "bundle" that will show the proper syntax available. I've never had problems with 100+ files open, either.
There are bundles which make Textmate usage fall far beyond that for just a programmer. You can set up Textmate to post to your blog, or as todo lists mimicking the style of Taskpaper, for instance.
Subversion and other version control systems are also supported, which is a huge help. If you like the indepth abilities provided by tools such as VI and Emacs (a bitter rivalry!), but want the aesthetics of the Mac platform included, Textmate wins on all levels.