Most z80 IDEs are no longer actively being developed or supported. The exception is Cemetech's SourceCoder 3 IDE, which lets you write z80 and ez80 assembly and assemble it directly from your browser. It also has the jsTIfied calculator emulator built in so that you can test your code without leaving your browser.
A no-longer-updated option is Zilog Developer Studio (ZDS). It can be found here.
While notepad (and other text editors) work fine, the advantage of an IDE is that it is easier to read (words are highlighted, etc.), and easier to organize (into a project). IDEs should have their own assemblers built in as well. And if you decide to do computer assembly programming, you can use the same IDE. Even if your IDE does not have an assembler built in, you can use download an assembler like Brass.
- With Notepad++, you can install an add-on to make the z80 text highlighted.
- Mimas 0.4 (the most recent as of 3/16/15 can be found here), is an on-calc IDE. Although it clearly has no text highlighting, the convenience of being on the calculator is pretty nice.