I like the free "myDevStudio for NSIS" editor. It knows about NSIS 2 (it was updated in March 2003). It has systax highlighting, folding, autocomplete etc.
The only problem I've come across so far is that it crashes immediately after I print anything - the hard copy seems to be OK but the program disappears. (So I never use the "Print" feature).
You can download it from http://www.vvvsoft.com/download.html
(the site also has some screenshots etc)