This page contains websites that host software for Solaris, especially older versions of Solaris. It includes free software and proprietary abandonware.
Unfortunately, most software for Solaris hasn't been backed up and is essentially lost to time. If you found something of note that isn't on this list, please let me know so I can add it.
Sunfreeware
The original Sunfreeware provided free software packages for Solaris, however they stopped providing packages and now require you to buy a subscription. However, there are mirrors of Sunfreeware available. The mirror I use is provided by Dokuz Eylül University. It provides most of the packages needed for basic usage, such as GCC, SSH, nano, Emacs, etc. Usually if I need a package I can find it here.
fsck.technology
One of the only archives that contains proprietary software, including applications such as Adobe Photoshop, WordPerfect, Internet Explorer, and more. Most of the software is for SPARC with some for x86.
OpenCSW
OpenCSW provides software as well as a custom package manager for Solaris 10 and 11. Unfortunately it doesn't appear to support older versions of Solaris, but it's useful when using Solaris 10 or 11.
ibiblio Solaris Package Archive
Hosts various Solaris packages from versions 2.3 to 11, mostly free software.
tgcware
This site hosts various pieces of free software compiled by the author for Solaris 2.6, 7, 8, and 9.
synack.net
Has software built for SunOS 4.
WinWorld
This website has a lot of software for a variety of platforms. You can filter applications by release platform, although it bunches all of the Unixes into one label. Here's a link to the applications list filtered to Unix. The search functionality is currently disabled as of writing, but you can use a custom Google search to search the website.
Internet Archive
This is essentially the last resort. Unfortunately the Internet Archive is not very well sorted and it can be difficult to find collections of software. Here is a link to a search for Sun Solaris software to get you started. Sadly most of the archives consist of only ISOs for installing Solaris rather than software to be installed on Solaris. You can also try searching for specific software or for Solaris software companion CDs.