Pronunciation of Finnish in a nutshell (for linguists)

The following table presents the basic phonetic values of letters in Finnish. Legend:

The first column shows a letter.
The second column gives the regular pronunciation using an IPA symbol.
The third column gives the regular pronunciation using Usenet IPA Ascii symbols, i.e. Ascii characters used as surrogates for IPA characters in contexts where the character repertoire is limited to Ascii. Warning: such notations have not been defined rigorously, so the information in this column is not very reliable.
The fourth column contains some notes on the pronunciation, including irregularities.

If you are not familiar with IPA notations, please see my description of Finnish pronunciation in plain (?) English.

a	ɑ	A	often described as [a] but normally [ɑ], due to opposition with [æ]
b	b	b	often close to or identical with [p]
c	k	k	[s] before e, i, y; occurs in foreign words only
d	d	d	dialects usually don’t have this sound
e	e	e	more accurately, [e̞] (lowered [e])
f	f	f	sometimes close to or identical with [ʋ]
g	g	g	often close to or identical with k; usually [ŋ] before [n]
h	h	h	[ɦ] between vowels; [ç] between front vowel and consonant; [x] between back vowel and consonant
i	i	i
j	j	j	after i, not pronounced in some contexts, e.g. lukija [lukiɑ] (usually)
k	k	k	no aspiration
l	l	l	[l̪] after [t̪]
m	m	m	often ɱ before [f]
n	n	n	often [ŋ] or [m] or [ɱ] depending on next consonant; [n̪] before [t̪]
o	o	o	more accurately, [o̞] (lowered [o])
p	p	p	no aspiration
q	k	k	no aspiration; in foreign words only
r	r	r	a trill
s	s	s	less sharp than English [s], due to lack of oppositions with [z] and [ʃ]
š	ʃ	S	often close to or identical with [s]
t	t̪	t	no aspiration; dental (unlike [d]); but alveolar [t] after [l], [r], [s]
u	u	u
v	ʋ	r<lbd>	approximant; sometimes [v]; usually [w] after a diphthong ending with [u]
w	ʋ	r<lbd>	in new loanwords from English, often [w]
x	ks	ks	(in foreign words only)
y	y	y
z	ts	ts	consonant pair, not an affricate
ž	ʒ	Z	often close to or identical with [s] or [z] or [ʃ]
ä	æ	&
ö	ø	Y	more accurately, [ø̞] (lowered [ø])

Additional notes:

the main stress is on the first syllable
secondary stress appears on the first syllable of the second part of a compound word, on the third syllable of a four-syllable word, and in many other situations
repetition of a letter indicates a long sound, e.g. aa is pronounced [ɑː] (unless e.g. an apostrophe or hyphen intervenes)
in foreign words, a double consonant letter may appear before a consonant but is then pronounced single (short), e.g. attribuutti [ɑtribu:t:i]
debatably, long vowels and especially long consonants are often described as identity pairs (pairs of identical sounds); this is common for consonants, since a long consonant has a syllable break inside it, and this cannot even be described if we write e.g. [ukːo] instead of [ukko] ([uk.ko]
two consecutive vowels often form a diphthong; the exact rules are hard to write down, but basically, there is a rich set of diphthongs that occur in the first syllable of a word, whereas later in an uncompound word, usually only i (sometimes u or y) forms a diphthong with the preceding vowel; denoting syllable boundaries with a period, we have e.g. suomi [suo.mi], kieli [kie.li], kauhea [kɑu.ɦe.ɑ], kauheissa [kɑu.ɦeis.sɑ]
the letter a is often described as being pronounced as [a]; since Finnish does not make a phonemic distinction between [a] and [ɑ], the sound value may vary within broad limits, but the need for distinguishing between a and ä makes it more natural to use [ɑ] in the description – though it is also natural that simple pronunciation instructions use the widely available letter a and not the phonetic symbol
ng is pronounced as [ŋː] (or as [ŋk] in some dialects, or as [ŋ] before a consonant or end of word in loan words)
nk is (usually) pronounced as [ŋk] and np as [mp]
the sound [s] in Finnish is really somewhere between [s] and [ʃ], but individual variation is considerable; moreover, /s/ may realize as [z] especially between vowels when speaking fast and sometimes as [x] or [ɹ̝] before [r]
the strong trill [r] is typical of Finnish, but there is considerable individual variation, and [r] is often replaced by the tap [ɾ] especially between vowels in fast speech
letters b, f, š and ž (as well as g except for the combination ng) occur in relatively new loan words only, and although the official pronunciation is as given column two above, they are very often replaced by “more Finnish” sounds
letter z, too, occurs in new loan words only, and even in them, a foreign z has usually been replaced by s or ts even in orthography (e.g., paleotsooinen); when preserved, it tends to be pronounced as [ts], although in the transliteration of Cyrillic letters according to a Finnish standard, it stands for [z]
other letters occur only in loan words which have more or less preserved the original orthography, and the pronunciation rules of the source language apply in principle; in practice, e.g. w is often pronounced as [ʋ] however
a large number of word forms (such as imperatives like anna and most noun nominatives ending with e) end in a vowel but are usually pronounced with a consonant after the vowel, effectively doubling the initial consonant of the next word, e.g. anna se [ɑnnɑs se], sade loppui [sɑdel loppui]; beware that this phenomenon, consonant assimilation at word boundardies, is often described wrongly or incompletely in textbooks, grammars, and other descriptions of Finnish pronunciation
Finnish pronunciation isn’t as regular as you might think after reading the description above, or any simple description of the language; there are actually quite a few irregularities, most (?) of which are described in Finnish on my page Onko suomen kirjoitusjärjestelmä ihanteellinen?

For comparison, you may wish to check the Rosetta Project page on Finnish orthography, which contains a pronunciation table as images scanned from the book The World’s Writing Systems. The table is simplified but correct, except for the letter v (and w). The table, as well as many other descriptions of Finnish, describe its phonetic value as the fricative [v], but the labiodental approximant [ʋ] is more correct.

The finer points in this description are largely based on the book Fonetiikan ja suomen äänneopin perusteet by Kari Suomi, Juhani Toivanen and Riikka Ylitalo (Gaudeamus, Helsinki, 2006).