Press ctl-k, then press one of these characters:

Character	Meaning
--------------------------------
!		Grave
'		Acute accent
>		Circumflex accent
?		Tilde
-		Macron
(		Breve
.		Dot above
:		Diaeresis
,		Cedilla
_		Underline
/		Stroke
"		Double acute (Hungarumlaut)
;		Ogonek
<		Caron
0		Ring above
2		Hook
9		Horn
=		Cyrillic
*		Greek
%		Greek/Cyrillic special
+		Smalls: Arabic, caps: Hebrew
3		Some Latin/Greek/Cyrillic
4		Bopomofo
5		Hiragana
6		Katakana

from http://vim.runpaint.org/typing/inserting-accented-characters/

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