Great recommendations!
Having recently made the switch to Vista, I also (re-)installed a bunch of useful and mostly free tools.
Already mentioned were
Foxit PDF Reader, a fast and clean replacement for the heavyweight Adobe Reader,
7-Zip for your compression needs,
Daemon Tools to mount ISO images and
Spybot Search & Destroy to find and remove unwanted ad programs.
µTorrent
I had Azureus once but found it too slow during startup and use and thus switched to the light utorrent.
Comodo Firewall Pro 3.0
On WinXP I used ZoneAlarm Free, but due to issues with that and utorrent, I switched to Comodo. Looks fantastic, works like a charm, and it's also free.
(
NOD32 Antivirus 2.7)
I was also using AntiVir PersonalEdition, but switched to NOD32, since there was a 1 year license in one of my c't magazines. They don't offer a free version, unfortunately, only a 30-day trial.
Notepad++
My notepad of choice. Reminds me of FoxitReader, kind of the same style. Syntax highlighting for lots of common file types.
XnView
It's very fast and does what I want my image viewer to do: view images in full screen, zoom with + and -, quit with Esc, go to next/previous image with mouse wheel or PgUp/PgDn. All without noticable loading times.
I loved ACD See back in the day when there was only version 3.0, which was fast, before they made a bloated suite out of it. I tried FastStone Image Viewer mentioned by Dark Savant and Moriendor too, some time ago, but couldn't get used to it, maybe due to the interface.
Free Download Manager
Great tool for downloading large files or many at once from a single website. You can click on a button to reduce download speed in case you need it for something else. In Firefox I can right click and select "download all files", set wildcards to "zip" and it would put all zip files in a queue and download them..
Looks and feels like FoxitReader too.
WinSplit Revolution
Got a large monitor? Don't know what to do with all your windows? Use this program. It lets your sort windows on your desktop on two sides, four or six corners, by a simple keystroke. No need to painfully resize every window manually. E.g. Ctrl-Alt-Numpad 4 for one window, Ctrl-Alt-Numpad 6 for the other and they are both perfectly aligned next to each other and take up all available screen space. (
Screenshots)
Oh, and last but definitely not least, already recommended by Gorath:
Total Commander
A file manager you use instead of Windows Explorer. Can't do file operations any faster than with this tool. It does take some getting used to if you only know Explorer, though. It's a bit like learning 10 finger typing when you are still a bit faster with two fingers. But eventually, it'll be worth it.
You can use it for ftp transfers, to synchronize directories so they have the same content, it offers a multi rename tool to easily rename lots of files at once, the list is endless.