As it is repeatedly discussed at work these days, I think it may worth mentioning there exists official locations to install bundled and unbundled packages, and well defined paths with a recommended purpose for most major players in UNIX and UNIX-like platforms such as Solaris, GNU/Linux and FreeBSD for example.
For Solaris (and OpenSolaris derived distributions), you can consult the Recommended Installation Locations for Solaris-compatible Software Components document from the Architecture Process and Tools community, along with the filesystem(5) manual page online, or on an installed system.
Although the BSD are not so well-known as GNU/Linux, there are some very good and alive projects. More, the documentation available for each distributions are well written, and frequently updated. For these systems, and most notably FreeBSD and HP-UX systems, you can read the hier(7) manual page online, or on a live system.
As a side note, it is interesting to note that Ian Murdock (founder of the famous Debian GNU/Linux distribution, and just joining Sun to head up operating system platform strategy) is also the chair of the Linux Standard Base—while Sun is a member of the Linux Foundation (which host the LSB project). Maybe can we expect more standardization between these two major players in the future?