Is there anything built-in that does this? (E.g. command line tools.) Or are there any third-party apps?
Update: I like The Unarchiver, but vote up your favourite, or add it as an answer if it isn’t there.
The Unarchiver really does its job well (and it's also completely free). From the site:
Supported file formats include Zip, Tar-GZip, Tar-BZip2, RAR, 7-zip, LhA, StuffIt and many other more and less obscure formats. The goal is to make The Unarchiver able to extract anything you give it. The Unarchiver uses the libxad unarchiving library to support many older, obscure formats.
Nothing built-in if I'm not mistaken (no access to a Mac now).
RAR has a command line version for Mac OS X. UnRarX is a GUI program to extract RAR files.
UPDATE: UnRarX is now deprecated and its site is down.
As far as well-regarded, RARLab's commandline for MacOSX is the best. It is not free, but there is free (-as-in-beer) official unrar tool (source code available here) that supports all RAR formats. You may be able to find a precompiled version somewhere if you don't want to compile it yourself.
There's also a free-as-in-speech unrar tool but that doesn't support the latest RAR3 format.
MacPar Deluxe will handle RAR files quite easily....
MacPAR deLuxe is a utility program that runs on the Apple Macintosh. It is useful to you if you download (or upload) binary files from internet newsgroups (a.k.a. “usenet”). Often, binary content comes in the form of sets of many files that together form a “rar” archive. MacPAR deLuxe assist you in combining these files after the download finishes. •It verifies that the file set is complete, and all files are undamaged. MacPAR deLuxe can recover missing information by processing so called "par" and "par2" files. •After the verification step, MacPAR deLuxe unpacks the data. If possible with its built-in unrar engine, or otherwise by launching the appropriate program.
The program automatically cooperates with Loek Jehee's Split & Concat. Features •Full support for the "par 1" and "par 2" standards. •Perform verification, recovery and automatic unrar (if applicable) by just double-clicking the ".par" or ".par2" file in the Finder. •Get a clear overview of the status of all files (see example screenshots). •Create a par or par2 volume set by dragging files to the window and simply save the document. •Open a rar archive and unpack the files inside. •Can automatically start an external program to process files in the par set. •Can open multiple windows and run multiple jobs at the same time. •Alternatively, multiple jobs can be queued and processed one after the other, to save machine overhead. •Many configurable options. •Fully compatible with "other" par and rar utilities. •Extensive documentation included via the "Help" menu. •Shareware: you can make make a donation if you like the program.