There are variants of the Keil editor which expand tabs to 4 columns. Makes a real mess of assembly language, which has always been formatted with 8 column tabs for as long as I have been alive. Some versions (not all) of uVision allow this to be fixed.

I use VIM, a VI clone with syntax highlighting. Since I work on several different projects, moving back and forth between linux, Windows and even Dos, I find it useful to be proficient with an editor that runs on just about any computer you are liable to encounter.

That proficiency came at a price. Learning VI is no picnic, and no friend of yours will ever recommend that you learn it. That said, if you decided to learn it just for the challenge of it, you might then find IDE-type editors hopelessly underpowered.

Another open source editor is EMACS. While I have never used it myself, those who do swear by it. At minimum, your editor should support macros, regular expression search and replace, syntax highlighting, and if programming in C[++], it should also support autoindent.