Let's see... to me there are three main categories:
Apps I use on a daily basis:
PStart - For me, this was the find of the century because I hated keeping shortcuts, to programs located on removeable media, on my Start menu. PStart allows me to completely separate out all my portable apps. The only thing that is missing for me is the ability to have a similiar hierarchical list of URL's (favorites if you will), but the notes feature is an added bonus.
Popman - This program has an essential role in my email handling strategy. It is a pop3 checker and I use it as a first line of defence against bogus or pointless emails (i.e. forward this to ten of your mates type emails). I am fortunate enough not to get tons of spam so I can use this to quickly delete out those emails that don't even warrant being loaded into my main email client. It allows me to check multiple accounts and I like how you can group those accounts into custom tabs. Great app.
KeePass - I used to use Pirem TopSecret but it isn't strictly portable and is no longer being developed so I'm in the process of switching to KeePass. With the myriad of forums I belong to I definately need something to keep track of all the logins and passwords I have!
ClipX - I used to use Clipomatic from Mike Lin but I don't think this is portable and it only supports text. ClipX not only supports text but also images as well. For the most part the functionality of each program is the same and, in general, is a utility that I wouldn't be without. I mainly use the "Stickies" feature rather than a clipboard history (although that can be handy when working on any sort of code). The stickies feature allows me to store frequently used strings of text such as HTML tags, signitures, usernames or anything else that I might want to paste frequently.
DM2 - I discovered this program on PF.com when I was looking for a portable folder bookmarking utility. I had previously used
DirKey but this is not a portable app. DM2 has the added bonus of having other functions like altering window transparency and minimizing windows to desktop icons. The great thing about this app is that it kills two birds with one stone and does it without becoming bloated and overly slow. I was looking for a window state manipulation tool (like altering transparency, always on top etc) and the fact that DM2 had it, plus the folder bookmarking is great.
Irfan View - Probably one of the most well known pieces of freeware on the planet, this is a truly awesome and useful program. I love it and seem to use it all the time, even if it is just to view images (since it is really fast). The image manipulation features are great and I frequently use them when working with images, thus avoiding the need to load a full on image editing program.
Apps that I use on a weekly basis:
YAMC - Yet Another Mail Client is a lightweight, yet comprehensive email client. I have become increasingly disillusioned with bloated and slow email clients that have tons of useless features that I don't need. YAMC seems to do the job and while it is not without its bugs (it's still under development), it does its thing without taking an age to do it.
HFS - I find this program infinitely useful to transfer files, over the internet, from one computer to another, without having to use unreliable messaging clients, slow email attachments, or other protocols that could potentially be blocked over some network. Since this program uses the HTTP protocol, it works pretty much all of the time. It is very quick and easy to set up and I can have files shared within seconds of loading it up.
FreeCommander - I like this Windows Explorer replacement not just because it has the dual pane feature but because of the small features built in to it. I commonly use the wipe feature to securely delete files and much prefer using it to copy/move files over the standard Windows Explorer. All in all it's just a better experience when doing some laborious tasks with large numbers of files.
FoxIt Reader - Why download Adobe's Acrobat Reader which is a 20MB+ download, when you can download a program <2MB to do exactly the same job? I'm almost offended by Adobe's inability to produce a small, simple program to view PDF files. FoxIt Reader does exactly what I need it to, without; taking days to load, installing plugins that I blatently don't need and having a stupid internet updater that never works properly.
Apps that I use infrequently but consider essential:
FileZilla - I used to use CuteFTP as my FTP client but it was not free and sometimes seemed a bit clumsy to use. FileZilla is intuitive, efficient and a joy to use. My only complaint is that it seems to be a tad big but it does its job and it does it extremely well.
DiskCleaner - There are tons of junk/temp file cleaners out there and I have settled on this one for some time. I have never had any problems with it and it has never caused my system to become unstable. I love the selective cookie deleting feature so that I can maintain cookies that keep me logged into forums while deleting those cookies that I don't need anymore. The program is expandable via plugins and is a must.
OrphansRemover - Great program for deleting dead shortcuts from your Start Menu and desktop. Being that I frequently try out new software and constantly install and uninstall stuff, this program comes in handy in keeping everything tidy.
SIW - This is an amazing tool, that has so many features that I'm still discovering new ones now, a month after first using the program! Granted this isn't a program that I use on a regular basis but it has so many important system tools and generates so much useful information that I know one day I will be thankful for this program when the obligatory bi-annual Windows failure occurs.
I have other apps that I use on my portable HD but this, I would say, is probably the pick of the bunch... besides which, this post must be huge by now!