Most commercial software that ran on 4.x either will run in BCP mode, or is available for Solaris 2.x, or is being ported now. Solaris 2.3 BCP mode finally supports statically-linked executables. Solaris 2.5 BCP mode supports mixed mode (part static, part dynamic) executables too.
You can obtain a list of official 3rd party porting commitments, maintained by Sun's "Solaris Demand Center" (whatever that is), by sending electronic mail to "sparc_products@thegift.sun.com" -- this is an automatic reply server. The list shows what third party applications are currently available for Solaris, and lists expected dates for many more.
Sun's web pages contain a searchable index of commercial programs and a link to an outside contractor who gathers data on free and public domain programs.
A list of freeware (some "public domain", but mostly copyright- but-freely-distributable) [as well as commercial software??] that has been ported to Solaris 2.x is posted monthly to the newsgroup comp.unix.solaris by ric@coronacorp.com (Richard Steinberger). Look for this:
Subject: Solaris SW list. Monthly Post.
Some software that invariably needs minor tweaking after an OS upgrade is included here specially. It's almost always necessary to recompile it after an OS upgrade, but if that still doesn't give a working version, make sure you check the archives for the latest version:
Top - a process monitor Site carrying the latest version of Top
Lsof - list open files
Identd - a daemon that implements RFC1413
scsiinfo - a program that lists SCSI devices.
sysinfo - a system hardware information program.
If you use gcc, it is important to remember that you must re-run fixincludes or re-install gcc after an OS upgrade or you'll be compiling with the old include files which will essentially give you the above programs as if compiled for a previous OS release.