A certain well-featured text editor has for a long time had a really nifty
feature, wherein when the user types the closing half of a grouping symbol
(mainly ')', '}', and ']', but the mechanism is extensible), the software
attempts to determine which opening-symbol it matches (if any) and briefly
flashes or highlights that symbol.  In case of a mismatch (e.g., the nesting
is wrong or '(' is matched up with '}') the highlight color is different 
(red by default IIRC) and optionally it can also beep at you.

There is also an extension available for this editor that allows the user 
to place the cursor before the closing or after the opening symbol and have
the entire region enclosed by the marks highlighted.

Whenever I'm typing anything of any significant length into OpenOffice, I
miss these features horribly.  I have to visually scan over the entire text
periodically, counting grouping symbols (especially parentheses) to make sure
I've closed off all the ones I should have and everything.  Sometimes I'm
tempted to type stuff up in a text editor and then copy and paste it into
OpenOffice, because of this issue.

I know some people would just turn such a feature off, but I suspect I'm not
the only one who would find it a useful timesaver.