The extension repository
Or TER for short is the place where every TYPO3 extension developer can upload their own extension for everyone to use.
It has a lot of extension and quite a few of them are listed as in beta. This is however not always a sign that the extension is not ready for use, an indicator of a good extension is the download count, the more downloads the more likely it is that the extension is good.

Feedback
If you have questions, find errors or just want to say hi, feel free to send me an email:
morten(at)nisker.dk

Navigation through the vast amount of extensions developed for TYPO3 can be quite a tedious task, to help people more easily find usefull extensions I have compiled a list of extensions that I find usefull and which I am using myself, both for this website and for websites at work.

fdfx_be_image
Backend image processing tool

tinymce_rte
An alternative to the built in editor. However since TYPO3 version 4.4 I stopped using this as the built in editor has gotten a lot better.

truncate_cache
Clears all caches by truncation instead of deleting, this makes it much faster on big websites.

t3quixplorer
a very nice tool if you do not have filesystem access on the server where you host your website.

mono_ga
Easy way to add google analytics to your site.

mc_googlesitemap
Easy way to add a google sitemap to your site. But take note that it will also show pages that are hidden in menu. This can be fine if it a page that is accessible via a link on a page anyway - But if you have page that you're working one hidden away by hiding them in the menu - google will index them!

pp_pagemetadata
requries pp_lib, lets you add metadata very easy, but check the configuration, the default settings needs trimming.

sp_socialbookmarks
neat little extension for adding social bookmarks to your pages, very configureable and extend able. Has a very neat click statistic feature.

daimi_responsible
my own extension for displaying who is responsible for a page and when it was last edited, has a lot of settings to let control what to display and how.

realurl
A must have for getting pretty URLs instead of index.php?id=123

danp_realurlconfigurator
Makes it a little easier to configure realurl

phpmyadmin
phpMyAdmin as a backend module.

templavoila
Templating system, which also add nice features like referenced content.

extensionlist
Nice little backend module that lists on which pages different extensions are installed. I can give a quick overview over what is used where. It does however not show you anything about what might be used via a Typoscript template.

heatmap
A simple tool for showing where users click on your page. It's by no means perfect and does not take different browserwindow size into consideration - but It can give you a fairly good picture none the less.

http://typo3.org/extensions/repository/view/heatmap_pro/current/