Compose Key cheat sheet
Use your Compose key then the Keys from the table below to get the Result character.

Keys		Result	Description
"	,		„		Czech opening quotes
"	<		“		Czech closing quotes (also English opening quotes)
"	>		”		English closing quotes, use ' instead of " for single quotes
␣	␣				No break space
-	-	␣			Soft-Hyphen (in-word break)
-	-	-	—		em dash
-	-	.	–		en dash
<	-		←		Left arrow (also - > for →)
<	<		«		Left guillemet (also > > for »)
.	<		‹		Single left-pointing angle quotation mark (also . > for ›)
o	o		°		Degree sign
s	o		§		Section mark
P	!		¶		Pilcro/paragraph
<	3		♥		Black heart suit
:	)		☺		White smiling face
:	(		☹		White frowning face
^	.		·		Middle dot
.	=		•		Bullet

e	=		€		Euro (also using C =)
l	-		£		Pounds sterling (also using L =)
c	/		¢		Cents

x	x		×		Multiply (times)
-	:		÷		Divide
-	,		¬		Not sign
<	=		≤		Less or equal
>	=		≥		Greater or equal
=	/		≠		Not equal
+	-		±		Plus minus
8	8		∞		Infinity
1	4		¼		Quarter (similarly half, various quarters, fifths, eighths…)
^	2		²		Superscript 2 (also 0, 1, 3, 4, 5, +, =, (, ), …)
_	2		₂		Subscript 2 (also dtto)
(	2	)	②		Circled digit 2 (similarly for numbers ⓪ – ⑳)
%	o		‰		Per mille sign
N	o		№		Numero sign

(	c		©		Copyright       (formerly: c o)
(	r		®		Registered      (formerly: r o)
t	m		™		Trademark

#	b		♭		Music flat sign
#	f		♮		Music natural sign
#	#		♯		Music sharp sign

s	s		ß		Eszett/digraph s, sharp s
o	e		œ		ligature (similarly ŒÆæ…)
m	u		µ		Mu/micro (also using / u)

'	a		á		apostrophe	→ acute				áćéíĺńóṕŕśúýźÁĆÉÍĹŃÓṔŔŚÚÝŹ…
`	a		à		grave		→ grave				àèìòùÀÈÌÒÙ…
,	c		ç		comma		→ cedilla				çşģķļņÇŞĢĶĻŅ…
"	u		ü		quote		→ umlaut/diaeresis		äëïöüÄËÏÖÜ…
^	a		â		circumflex	→ circumflex			âêĥîĵôŝûÂÊĤÎĴÔŜÛ…
~	a		ã		tilde			→ tilde				ãñõÃÑÕ…
*	a		å		asterisk		→ ring above			åůÅŮ…
.	a		ȧ		period		→ dot above			ȧėḣṁȦĖḢṀ…
/	o		ø		slash		→ stroke (bar)			đħłøƶĐĦŁØƵ
c	c		č		c			→ caron				ǎčďěǧȟǐǰǩľňǒřšťǔžǍČĎĚǦȞǏǨĽŇǑŘŠŤǓŽ…
<	c		č		less			→ caron (not for all?)	(dtto)
_	a		ā		underscore	→ macron			āēḡīōūȳĀĒḠĪŌŪ…

I kind of miss the way to compose these symbols:

	minus	−	U+2212	(Ctrl+Shift+U 2 2 1 2 Enter)
	l-r arrow	↔	U+2194	(Ctrl+Shift+U 2 1 9 4 Enter)

Resources:

	http://linux.m2osw.com/compose-key-under-linux
	http://www.astro.ufl.edu/it/docs/intl-keyboard.html
	http://en.wikipedia.org/wiki/Compose_key
	http://fsymbols.com/keyboard/linux/compose/
	https://help.ubuntu.com/community/GtkComposeTable

(Tested on XUbuntu 13.10, June 2014, macek@sandbox.cz)