When learning a language it is nice to do it the hard way, so you take the default txt-file editor provided with your OS. No colours, not help, no nothing, pure hard-core learning. But in Linux-desktop Gnome the default editor Gedit is quite powerful without doing too much, has an official Windows-port and has a OSX Darwin-port. It took just a few hours to understand how highlighting in Gedit works and to get it implemented. I got some nice help from the work done at the cuda-highlighter by Hüseyin Temucin (for showing how to extend the c-highlighter the best way) and the VIM OpenCL-highlighter by Terence Ou (for all the reserved words). This is work in progress; I will tell about updates via Twitter.
Windows-users first need to download Gedit for Windows. OSX-folks can check Darwin-ports. Then the files opencl.lang (.cl-files) and opencl_host.lang (extension of c to highlight OpenCL-keywords) needs to be put in /usr/share/gtksourceview-2.0/language-specs/ (or in ~/.local/share/gtksourceview-2.0/language-specs/ for local usage only), or for Window in C:Program Filesgeditsharegtksourceview-2.0language-specs or for OSX in /Applications/gedit.app/Contents/Resources/share/gtksourceview-2.0/language-specs/. Make sure all Gedit-windows are closed so the configuration will be re-read, and then open a .cl-file with Gedit. If you have opened cl-files as C or Cuda, you have to set the highlighting to OpenCL manually (under view -> highlighting). For host-code you always need to set the highlighting manually to “OpenCL host”. You might want to associate cl-files with Gedit.
StreamHPC is working on Eclipse-support and I’ve understood also work is done for Netbeans-support. Let me know if there are more alternatives.