I have found many essential OS/2 utilities. Here's links to many of them.
File Commander/2 is an excellent clone of Symantec's Norton Commander for DOS. This program goes beyond Norton Commander in many ways, though. It allows you to search for a file from the current directory or from root. It also interacts well with the WPS and supports most of the Norton Commander keyboard functions. It will even go into archive files like Norton Commander!
Monitor is a program that I wrote as my own CPU monitor. This one may not make use of the system calls now available to obtain CPU utilization, but the sub-idle priority option works about as well. This monitor is small and graphs instantaneous, average, and average over the last hour CPU usage along with an instantaneous percentage and loop count. The loop count is a somewhat useful benchmark as it scales with CPU performance. The graphs on this are carefully set up so it shows usage over the last minute and hour. There are options to set colors of different fields and to stay on top. All settings are saved and position is remembered. Check out the smooth dragging when moving it around the screen. This was my first real PM program. Source is available upon request.
Object Desktop is very useful for me. The virtual desktop is one of the best I have used. The tabbed launchpad is a convenient way to organize frequently used programs. It also adds lots of little things to the desktop, such as enhancing the look of folders and the task window. Some features are subtle but useful. The ability to open archive (ZIP, ARJ, TAR, ARC, LHZ, etc) files as folders is useful, and the folder caching capibility improves performance for those folders I frequently open. The Pro version adds a tape backup program (IDE only) and the ability to view almost any file format seamlessly. It also provides a method of packaging folders into files to move them to another system or just to back them up. Some people have reported instability problems with this, but I have not had any problems. Users of OS/2 version 3.0 will appreciate the immediate benefits of this more so than Merlin users, but I still use it frequently.
Process Commander is an excellent program for stopping those wayward programs. It has tons of features and is basically Watchcat on steroids. It even has remote control access for those on a network.
NPSWPS is another nice WPS enhancement program. This one is free and seems to work well with Object Desktop. NPSWPS adds a bunch of small features. These include activating the window under the mouse (without bringing it in front) like X windows. It also adds things like automatically positioning the mouse cursor for dialog boxes and animating window opening and closing. It also adds the mouse action capability to OS/2 3.0. This is an excellent utility.
Post Road Mailer is a good email program. I cannot compare it to PMMail as I have not used it, but Post Road works quite well. The filter feature is nice for filtering out spam, especially since it can scan the entire headers for words like "Cyber Promotions" or "ispam.net" and forward the mail right back to them. It also will automatically filter email and drop it into the appropriate folder set up. For example, it can put mailing list emails into the appropriate mailing list folder. Double clicking on a URL in the email brings up Netscape. It also interfaces with PGP, virus scanners, and Spellguard, although I havn't been able to get the spell checking to work properly yet.
Adobe Acrobat Reader for OS/2 is available in beta for OS/2. Acrobat is used to read all those .pdf files. The OS/2 beta works reasonably well and automatically plugs-in to Netscape Navigator for OS/2 for seamless pdf viewing. The January beta has problems with the current Netscape, however. It sounds like a new beta may be available shortly from an email response I received about a bug report I submitted.
And who can forget Netscape Navigator. If you havn't been living under a rock you know that Netscape is available for OS/2 with a lot of multimedia plug-ins. There are also numerous 3rd party plug-ins for things like Quicktime and VRML. Netscape is currently working on porting Netscape Communicator to OS/2.