The best, IMO is the Protean IDE, which has been released for free recently.
http://www.proteanide.co.uk/index.php?pid=downloadsIt just makes programming so much easier with pop-up command help (and even for your own functions too), code completion, bookmarks,
multiple undo, cold folding, EXE icon replacement, project management, multiple syntax highlighting themes (I have never really liked Blitz's default syntax highlighting), etc., etc. What's nice about it is that all those features are not in your way, like some IDE's, but always seem to be there just when you need them. My favorite features are:
Code completion (or whatever it's called), which shows you a list of options as you type. For example, if you type "tank\sp" in my project, it will automatically display "tank\speed" on a pop-up list.
Function help: When you type a function, a little help box pops up below your cursor (not obstructing your typing), with a short description of the command, and which argument you are currently typing.
I know that many of it's features you may never even need, but it's always nice to have them when you need them.
I just stick to the normal blitz IDE. No real reason to switch unless.
*Except some functionality like Icons.
If you try Protean IDE, I think you'll change your mind. :)