Have you ever visited http://www.nirsoft.net/ ?

There are many powerful tools in it. I will list down several of my favourite utilities from NirSoft:
RegScanner - Search the registry key, provide very good interface. You can this instead of the “Regedit.exe” built-in search.
VideoCacheView - This one is really powerful, which is recently added. If you are watching online video, the .flv file, then after finish watching, the video is downloaded in your cache. And using this utility, you can save the cached video to your folder. I haven’t try it with youtube, but I tried it in other websites using IE, and it works.
CurrProcess - I use this in order to replace Windows task manager before. But, finally, I stick back to task manager. However, this utility really provides a lot of information of the current process. Really nice.
ServiWin - This utility will display the installed drivers and services, and you can disable them using this utility.

I think most of the people switch IE to Firefox. However, I use Firefox as the complementary application for browsing. I use portable Firefox, extract once and use anywhere. However, it is slow to start the application.

But, Firefox is really nice.

The followings are some extensions/plugins/addons that I like:
Adblock Plus - Disable a lot of advertisement, so that, no need to download those unwanted advertisement.
All-in-One Gestures - Using mouse gesture to “Go back one page” and “Go forward one page”, how nice? Just by draging right-click from right to left or left to right.
All-in-One Sidebar - You can view all your addons at the sidebar, easy and clear.
Clear Cache Button - A button at the toolbar, click once clear your cache.
CookieSwap - Just, if you want to visit some page, but you need to clear the cookies, however, you want your cookies remain, then, you swap a new profile using empty cookie. After that, you can swap back your original profile, and your cookies are still there.
Download Statusbar - Overload the default download interface, very nice.
Download Helper - You can sniff the .flv file from the website such as YouTube. Very nice.
DownThemAll! - Download accelerator, so that you can download the file faster.
Fasterfox - Though I install it, I not sure whether it affects the browsing speed or not.
FAYT - Find as you type, very good, no need to press “F3” or “Ctrl+F”, just type and you start find the text in the page.
FEBE - Backup your Firefox extensions.
Firebug - Are you writing your own webpage? Are you going to use Javascript? If you are using Javascript, then this extension is very nice for you.
Flashblock - If you visit a webpage and there is a lot of Flash, then it will affect your download speed, because your connection is downloading those Flash. How about disable them, and if you want, just click on them?
FlashGot - Do you have any download manager such as Free Download Manager, Orbit and FlashGet? You might like the download manager integrates with your Firefox.
Greasemonkey - How to use this extension? You need to install the userscripts, then you can do many things. Eg: change the text and background colour, resize the textarea. Because it uses Javascript. If you good in programming, you can write your own script and modify the page you are browsing.
Image Toolbar - Move your mouse to the image, popup a toolbar, then you can save, copy and else, just like IE.
MediaPlayerConnectivity - Do you feel frustrated when you find a song in Chinese websites, but you cannot play those .wma file? By using this extension, you can connect the media file to your other media player you have installed. And it will not start the media file automatically unless you click them.
Mozilla Archive Format - Using IE, you can save the page as .mht, so that all the image files will be included inside one .mht file. How about Firefox?
NoScript - Script are dangerous? Using this extension, disable annoying script.
PDF Download - Annoyance when open a PDF file because it will embedded in the web browser, sometimes just makes your browser no response?
PopupMaster - Disable annoyance popup.
Quick Preference Button - Want to change your Firefox preference? Eg: you can disable image easily with this extension.
QuickRestart - Click and restart your Firefox. Because, after installing the plugins, you might not restart immediately.
Session Manager - Firefox suddenly terminated? How is your previous browsing pages? Where are they?
Show Picture - Visit a page, and the image is slow to show up, or just no response. Then, you can right-click and “show picture” just like IE.
Tab Mix Plus - Tab browsing, with more other functions.
Uppity - Just click the button, then you can go up one level of your url address.
User Agent Switcher - Change your browser’s user agent.
View Dependencies - View the information of the page, the dependencies.