Short version: If you want to open a StuffIt (.sit) archive on Linux, you're out of luck, because none of the tools can deal with recent StuffIt compression methods.
StuffIt compressed archives (characteristic filename extension = .sit) are frequently distributed by Apple Macintosh OS users. They are analogous to gzipped tarfiles (or ZIP archives). StuffIt for MacOS first combines the Mac files' data and resource forks, and then subjects the combined file to Lempel-Ziv compression. (The latter is exactly the method the old Unix "compress" program used, the one that has been supplanted by the patent-free gzip utility.)
Most people will advise Linux users to download Aladdin
Systems's "freeware" (proprietary gratis-usage i386-Linux-only
binary) program StuffIt Extractor.
Note: The rather old v. 5.2.0 Stuffit Expander utility available there has an obnoxious tendency to segfault on modern Linux distributions before it can accomplish anything useful. Late note: Correspondent Ambrose Li reports that the old version (in the _old directory in the ftp site) actually works. At least for opening old StuffIt archives, that may be useful, unlike most other options detailed below.
The open-source option is the "unstuffit" and "unsit"/"sit" utilities by Nigel Perry and Allan G. Weber, respectively. If your Linux distribution doesn't yet package them, you can find them here:
The Nigel Perry "unstuffit" utility is a bit difficult to find, since most search-engine hits will find the Aladdin Systems proprietary StuffIt Extractor, for some reason, instead. However, it's worthwhile persevering, since Perry improved on Weber's "unsit" code.
The bad news is that the December 1990 unstuffit/unsit utilities referenced above cannot read the compression algorithm characteristically used in modern .sit archives.
The open-source "macutil" package includes the "macunpack" utility, a further modification of Perry and Weber's unsit/unstuffit code. Unfortunately, even thought it is arguably more solid than those two programs, it is likewise unable to deal with the compression format in modern .sit files.
("macunpack" will also unpack PackIt, Diamond, Compactor/Compact Pro, most StuffItClassic/StuffItDeluxe, and all Zoom and LHarc/MacLHa archives, and archives created by later versions of DiskDoubler. Also it will decode files created by BinHex5.0, MacBinary, UMCP, Compress It, ShrinkToFit, MacCompress, DiskDoubler and AutoDoubler.)
So, unless you're trying to handle really old .sit archives, there currently is no real solution I know of on *ix platforms (other than MacOS X). Your best option is to coax your MacOS-using friends to repack (newer) .sit archives in ZIP or gzipped-tar format.
More information at:
An indirect and more-creative approach, however, suggests itself that can solve all of the above problems: On x86 Linux, if you install ARDI Executor ( http://www.ardi.com/ ), you suddenly have an excellent emulation environment to run single-tasking MacOS 7.x-type applications — such as StuffIt Extractor for MacOS. This should be a comprehensive solution (for Linux users on x86.)
2008 addendum: ARDI Executor now has an open-source semi-rival, that (within several x86 operating systems as host environments, including x86 Linux) can support running MacOS 7.x/8.x by emulating the 680x0 CPUs and related hardware: Basilisk II. However, unlike ARDI Executor, Basilisk II requires you to furnish (1) a Macintosh ROM image file, and (2) a copy of Macintosh OS 7.x, 8.0, or 8.1. (Macintosh OS 7.5.3 can be downloaded free of charge from Apple, Inc. The standard method for generating a ROM image without committing copyright violation is to acquire, say, an old Macintosh Quadra machine and use something like the RomGet utility provided inside the SoftMac ZIP archive, booting that utility on a floppy to read the machine's ROM and thus create an image file.)
From: Rick Moen email@example.com
Date: Mon, 3 Feb 2003 03:23:41 -0800
Subject: Re: [ILUG] DMG reader
Quoting Laur Ivan (firstname.lastname@example.org):
> I banged my head against a wall with this one: Is there
a DMG (Disk
> Copy image on macs) tool for intel computers? My problem is: I have a
> bunch of icons/themes etc files for macs and all are .dmg. All my
> googleing failed lamentably with results like "if you want to read
> them, get "Disk copy" from Jaguar... :(.
2002-09-10 11:50 / .dmg and linux?
"The ability to distribute disk images (.dmg) files is currently exclusive to Mac OS X." The core hdiutil - used to manipulate (create, convert, attach, burn, ...) disk images - isn't open source yet, partially because of some proprietrary codecs it uses. There's been a request  on darwin-development for it to be so, with an encouraging response  but nothing definite was said. So .dmgs aren't even available in Darwin without OS X, let alone linux, windows, or, I'm pretty sure, OS 9. (It's barely worth mentioning how short-sighted it is to have an OS specific means of distributing software, and that even more so for plain old source code.)"
Username/password for the above list-archives is currently set to archives/archives , which password is said to be there to foil spammers' address harvesters.
If you were talking about the analogous .img files from earlier MacOS, the remedy would have been to install ARDI Executor (http://www.ardi.com/ ) on your x86 Linux box, and perform the extraction work there. That is also the standard answer to the question of how one unpacks recent-version StuffIt (.sit) and Stuffit Self Extracting (.sfx) archives under x86 Linux ( http://linuxmafia.com/faq/Apps/stuffit-archives.html ).