...in other words, what are the absolutely necessary applications that you should install first on a fresh OS, or better, merge in your installation "master" (using nlite, vlite, PE Builder, ...) ?
Here's my top-10 list:
Avast Antivirus: I've been relying too long on my corporate virus protection until I rescanned some of my old backups with the free Avast edition and discovered how many viruses had just been left undetected. To me, this is one of the best and least invasive Anti-Virus, along with Kaspersky probably.
Orbit downloader: It's often faster to just go and re-download stuff than retrieving them from your archives. This powerful downloader (better than FlashGet and FreeDownloadManager imho) is particularly welcome for users with slow or unreliable Internet connection. It doubles as an excellent streaming media grabber utility.
7-Zip: one of the most quickly needed tool is a good compression/decompression utility. This one is my favourite. It supports most formats for packing and unpacking (7z, Zip, GZip, BZip2, Tar) plus a bunch of others for unpacking only (Rar, War,...). Context menu can be arranged to your taste for faster handling of the archives.
Notepad++: You'll need to edit config files, and check logs in the early hours of an OS installation
Miranda IM: this multi-protocol instant messager replaces yor traditional ICQ, GTalk, MSN, ... in one program. Its force resides in its plugins. It takes some time at the beginning to setup Miranda with your ideal mix of plugins and interface customization, but it's worth it! (for non-Windows users, Pidgin is another nice alternative)
KeePass Password Safe: this well-named freeware is simply the most clever and complete password database featuring strong encryption, auto-type function, rich import/export, plugins, and a portable version. The database can be used on J2ME, Blackberry, iPhone, Windows Mobile, PalmOS, Android, Linux and MacOS thanks to a collection of 3rd-party companions.
Startup Control Panel: This small tool will help you to keep under control the applications that slow down your system at boot time by stacking in the various autorun locations.
TweakUIXP: a utility to safely customize and tweak your system without having to put your bare hands into the registry.
CCleaner: A safe system cleaning utility that can be launched from you recycle bin shortcut menu. It will find the unused temporary files, registry entries left over by some careless uninstallers to keep your PC optimized. From the same house, see also Recuva for recovering deleted files.
myPhoneExplorer: That phone manager is absolutely necessary for any Sony Ericsson user.
And finally: it's not an application but a simple registry tweak: it allows you to launch the command prompt from inside any folder. As a result, you'll see this new item in your contextual menu: