(NOTE: I'm not really updating this list any more, but instead using del.icio.us. Hopefully soon I move all these over as well and "reunify". That said, there's still a lot here that might be useful...)

Below is a categorised list of free and/or open-source tools of particular use to developers. It should be noted that various restrictions may apply to the "free" version. Where I know about these I have added a brief comment, but you should read the licence details carefully before installing.

As this list is never going to be exhaustive, you can also check out the following "repositories" of open-source projects:

Freshmeat is very useful in this respect, as it allows you to search for open-source projects, see how other people have rated them, and most even get e-mail notifications when updates are released.

The MSDN C# DevCenter maintains two pages similar in intent to this one, with the exceptions that they only cover .NET technologies and languages, and not all of the tools are free. They are:

Also from MS is the PowerToys 'blog, and worthy of consideration (though not strictly in this category) are the Visual Studio Express SKUs, which are currently free in beta and CTP form, but which are expected to be sold for "a modest fee" when they hit RTM. (UPDATE: The VS Express packages have RTM'ed and are actually free to download for the first year. Even better news is that they are not "time-bombed" versions, so will remain free to use after the year is up. See Dan Fernandez's post for more details...)

For more "not necessarily free tools" there's also SharpToolbox.com and JavaToolbox.com, and Scott Hanselman's most excellent "Ultimate Developer and Power Users Tool List" (link is to 2005 edition).

Well, enough of the chit-chat, and on with the list itself...

If you have any other suggestions (for products, categories or "repositories") then please leave a comment.