tcomment provides easy to use, file-type sensible comments for Vim. It 
can handle embedded syntax.
TComment works like a toggle, i.e., it will comment out text that 
contains uncommented lines, and it will remove comment markup for 
already commented text (i.e. text that contains no uncommented lines).
If the file-type is properly defined, TComment will figure out which 
comment string to use. Otherwise you use |tcomment#DefineType()| to 
override the default choice.
TComment can properly handle an embedded syntax, e.g., ruby/python/perl 
regions in vim scripts, HTML or JavaScript in php code etc.
tcomment favours the use of line-wise comment styles. This implies that usually 
whole line will be commented out. tcomment also knows block-style and inline 
comments that can be used via special maps (see below) or the |:TCommentAs| 
command.
Demo:
http://vimsomnia.blogspot.com/2010/11/tcomment-vim-plugin.html
 *tcomment-maps*
Key bindings~
Most of the time the default toggle keys will do what you want (or to be 
more precise: what I think you want it to do ;-).
 *tcomment-operator*
As operator (the prefix can be customized via |g:tcommentMapLeaderOp1| 
and |g:tcommentMapLeaderOp2|):
 gc{motion} :: Toggle comments (for small comments within one line 
 the &filetype_inline style will be used, if 
 defined)
 gc<Count>c{motion} :: Toggle comment text with count argument 
 (see |tcomment#Comment()|)
 gcc :: Toggle comment for the current line
Explicit commenting/uncommenting:
 g<{motion} :: Uncomment region
 g<c :: Uncomment the current line
 g<b :: Uncomment the current region as block
 g>{motion} :: Comment region
 g>c :: Comment the current line
 g>b :: Comment the current region as block
In visual mode:
 gc :: Toggle comments
 gC :: Comment selected text
CAVEAT: If you visually select text within a line, the visual mode map will 
comment out the selected text. If you selected text across several lines, the 
visual mode map will assume though that you wanted to comment out lines -- 
since this is how many vim maps work. In order to make tcomment use e.g. inline 
comments anyway, use the <c-_>i map -- see below.
By default the cursor stays put. If you want the cursor to the end of 
the commented text, set |g:tcommentOpModeExtra| to '>' (but this may not 
work properly with exclusive motions).
Primary key maps for normal and insert mode:
 <c-_><c-_> :: :TComment
 <c-_><space> :: :TComment <QUERY COMMENT-BEGIN ?COMMENT-END>
 <c-_>b :: :TCommentBlock
 In insert mode, the cursor will be positioned inside 
 the comment. In normal mode, the cursor will stay 
 put.
 <c-_>a :: :TCommentAs <QUERY COMMENT TYPE>
 <c-_>n :: :TCommentAs &filetype <QUERY COUNT>
 <c-_>s :: :TCommentAs &filetype_<QUERY COMMENT SUBTYPE>
 <c-_>i :: :TCommentInline (in normal and insert mode, this map will 
 create an empty inline comment, which isn't suitable for 
 all filetypes though)
 In insert mode, the cursor will be positioned inside 
 the comment. In normal mode, the cursor will stay 
 put.
 <c-_>r :: :TCommentRight
 <c-_>p :: Comment the current inner paragraph
 <c-_><Count> :: Set the count argument (a number from 1 to 9) for use with 
 the subsequent tcomment map/command (see 
 |tcomment#Comment()|)
 E.g. in JavaScript, in order to get an empty /** */ 
 comment for documentation purposes, in insert mode type 
 <c-_>2<c-_>i
 In order to get an empty block comment like >
 /**
 *
 */
< type <c-_>2<c-_>b
Most of the above maps are also available in visual mode.
A secondary set of key maps is defined for normal and insert mode:
 <Leader>__ :: :TComment
 <Leader>_p :: Comment the current inner paragraph
 <Leader>_<space> :: :TComment <QUERY COMMENT-BEGIN ?COMMENT-END>
 <Leader>_i :: :TCommentInline
 <Leader>_r :: :TCommentRight
 <Leader>_b :: :TCommentBlock
 <Leader>_a :: :TCommentAs <QUERY COMMENT TYPE>
 <Leader>_n :: :TCommentAs &filetype <QUERY COUNT>
 <Leader>_s :: :TCommentAs &filetype_<QUERY COMMENT SUBTYPE>
... and for select mode:
 <Leader>__ :: :TComment
 <Leader>_i :: :TCommentInline
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Status: Works for me (there may be some minor quirks)
Install: See http://github.com/tomtom/vimtlib/blob/master/INSTALL.TXT
See http://github.com/tomtom for related plugins.