Ahhhh ... duh... my eyes must've been wiggin'! Could'a sworn you were talking about CFMAIL.
I think what you're after is an IDE (Integrated Development Environment) rather than an API. An API is generally a layer in the server code that allows hooks to perform custom code directly on the system at a lower level than can usually be accomplished by the interpreted language.
If that's what you're after, there really isn't one. CFX tags come the closest, in that you can run you're own low level code, but the extent of the actually ColdFusion integration is simply CFML calling that tag.
If you're looking for an IDE, there are a limitted number of tools that are specifically designed for ColdFusion. Dreamweaver is certainly one. Several of the guys on my team who swore they'd never use Dreamweaver are now working with it. From what I've seen, Dreamweaver MX allows you to strip out all the WYSIWYG and visual design stuff from the interface and present more of a source level editor. Kind of like Dreamweaver's "Edit source" window, but it's the default interface and can apparently be extended to be a much more robust code editing tool.
I've been contemplating making the switch myself... because even when you're running in this source editing model, you still have access to Dreamweaver's libraries of elements, etc.
Allaire's ColdFusion Studio was the defacto standard tool for editing CFML in CF 4.5. It was essentially the same engine as HomeSite (Allaire's popular HTML editor), but with an extra component palette or two. When Macromedia bought Allaire, and CF 5.0 was released, Macromedia unexplicably didn't release a 5.0 version of Studio (at least not on a wide scale... there are apparently a few builds that were available to special folks). So a lot of people moved to HomeSite 5 (which was released), and borrowed the CFML-specific palettes from Studio. You can usually still find HomeSite around. This is what I use now. It has pop-up tooltips and context-sensitive help, autocompletion of tags, attribute prompting for both HTML and CFML tags (when the CFML help files are installed), color coding of source code, and an integrated Tidy code sweeper to validate your code. It doesn't really provide for database or DSN management/validation and doesn't integrate directly with CF Administrator.
Other than Dreamweaver and HomeSite/Studio, you can certainly use any text editor or HTML editor you find. Most HTML editors allow you to define new tags yourself... so, with a little time, you could set them up with a library of CFML tags.
Or just use Notepad or Textpad.
-Carl