Here are a bunch of extremely useful shell plugins and extensions.

SageThumbs is the ultimate thumbnail previewer. It adds support to Windows Explorer thumbnails view for PSD(Photoshop), CRW(Canon RAW), AI(Adobe Illustrator), DCR(Kodak RAW),EPS (Encapsulated Postscript, MOS & NEF (Nickon RAW) and over a hundred other formats. Extremely useful now that PSD thumbnail support has been dropped in Photoshop CS1. The broad support for RAW files will please digital photographers, while support for Photoshop, Illustrator and Postscript will delight designers.
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One of the free power toys released by Microsoft, this on is by far the best. Right click on any folder in Windows Explorer and an "Open Command Window here" dialogue appears. Clicking it opens a command window, with the current working directory set to the folder you clicked.
Note: Microsoft has this utility for other versions of windows somewhere on their site.
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Free applications for Windows, their nature extremely useful.

A port of the core Unix shell utilities to Windows. Usefull if you find yourself in the command line alot, especially if you use both systems. Around 120 programs are in this package including diff, grep, md5sum, bison, head, tail, cat, ls, gunzip, wc, etc. align:left;
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XEmacs is probably the finest multi-purpose text editor available for Windows. IT has syntax highlighting and smart tabbing for just about every programing and scripting language out there. While fully configurable, its interface and baroque keyboard shortcuts will turn away some users, but those that persevere will be rewarded with an extremely efficient editing tool.
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http://www.burgaud.com/wscitecm.html
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Filezilla is pretty much the best ftp program I've used on Windows. Its free, open source, and easy to use. I used to use WS-FTP, but filezilla is much easier to use and less clunky.
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IrfanView is a free image viewer, whith support for a massive number of different image formats. Unlike the default windows image viewer it has support for lossles jpeg rotation, and raw file support. Dont forget to grab the plugins pack when your download it.
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MiKT_eX is a windows port of the TeX typesetting language and the LATEX document preparation system. The LaTeX system is particularly good at typesetting complex mathematical and chemical formulas. It can be used as a powerful presentation layer when combined with XML and creates beautifully formated documents. The system outputs PDF, DVI or PS files.
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TeXnicCenter is a front end for the LaTeX system. It integrates the TeX compiler, Tex2PDF, and a text editor with syntax highlighting and spell checking. Its a lot nicer then alt-tabbing between XEmacs and a command line shell all the time.
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Provided by Sysinternals, Regmon is a registry monitering utility. You can get it to filter by program and see what changes to a registry a programing is amking, and also see what registry keys it is reading.
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Another Sysinternals tool, Filemon works like Regmon, only monitering file access. A usefull debuging tool if you are having problems with file access.
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The third of Sysinternals keystone products, Process Explorer is a supped up Windows Task Manager. It allows you to peek inside the svchost.exe processes, and generally get a lot of info on what your system is currently running. A nice little trick you can use when explorer refuse to delete a file is search by handle for the file name then kill the process that has the lock on the file.
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The final Sysinternals program I will look at is Autoruns. Simler in premise to msconfig.exe it offers a lot more power. It lists programs that are auto-started with at windows boot time as well also dll's, Internet Explorer Browser Helper Objects and tool bars, Explorer shell extensions and more. Perfect for speading up your system or just disabling some anoying spyware.
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Putty is a small suite of tools for securely connecting to remote computers. Included are ssh, scp, and sftp clients. A good means to connect to home server without using insecure protocols such as ftp or telnet.
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Im vaguely reluctant to mention this program since unlike evrything else here it is shareware (nagware). TTF Manager is however simply the best font managment utility available. Out of around 30 font managers TTF Manager came out number one. I have around 1000 fonts on my computer, a situation that causes some dificulties when it comes to storing and viewing them. You obviusly dont want them all installed so TTF Manager makes it easy to install and uninstall fonts, while giving a drag and drop interface.
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I used to be a fan of WinRAR, then I discovered IZArc. IZArc is a fully featured compression utility. It handles all the standard file types (zip, rar, iso) and has full shell intergration. Its shell intergration is somewhat more friendly then WinRAR's.
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Win Merge is a visual diff program. It shows the differences between files, making it usefull for versioning, and finding places that files differ. When you need to compare files this tool is for you.
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The Firefox web browser is definitly the best web browser available at the moment. Support for tabbed browsing, popup blocking and an extensable plugin architecture make it a pleasure to use. You should be using this browser simply for the fact that your less likely to pick up spyware and viruses. If you do any web development the Web Developer extension will prove a boon, as will Foxylicious for users of del.icio.us.
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http://gisdeveloper.tripod.com/scite.html