OS X ships with a built-in Apache server, which you can enable by checking 'Web sharing' in the Sharing preference pane. There's a lot of other goodies like that under the hood, including Perl, Python, Ruby, and PHP. These languages may be out-of-date versions, and may require enabling. In general, OS X's popularity and uniformity make stuff like this easy to Google.
Unless you're a die-hard command-line person: For launching, file browsing, web searching, and more, try Launchbar. Quicksilver does much the same thing, but QS is poorly maintained (the original developer open-sourced it and walked away) and is now too slow and crashy to be usable on current versions of OS X. (YMMV.) Either one will move many common actions from the mouse to the keyboard in an easy-to-learn way.
For text editing, the two preeminent native-OS X editors are BBEdit and TextMate. Lots of arguments about which is better, and different language communities tend to have a favorite. Of course, if you're a Vim or Emacs user, you should have no problem getting your preferred editor up and running.
Consider also the following programs, which I haven't seen anyone else mention here:
* Hazel -- runs actions automatically when files are added to folders
* NetNewsWire -- RSS reader
* Jumpcut -- multiple clipboards
* QuicKeys -- attach sequences of actions to key combinations
* PathFinder -- powerful Finder relacement
* Acorn -- image editor
* OmniGraffle -- wireframing and flowcharts, very useful for web design