Romanaccio (or romanesco if you're being polite about it) is the (IMHO) hideous dialect spoken in Rome.
Some useful romanaccio phrases include:
'namo a magna', aoh = Let's go and eat.
(cf. andiamo a mangiare in standard Italian)
anvedi oh che roba! = What a surprise!
(can also be abbreviated as just anvedi oh! or indeed anvedi!
aoh is a surprisingly versatile word. it can be used as a greeting, for emphasis, and more generally to start or finish more or less any sentence. volemo er verde, aoh would be a good way to inform local politicians that you were in favour of public parks.
er is the definite article
all infinitives have the final -re removed and replaced with an accented letter (at least as far as the pronunciation is concerned) magnà, capì and so on.
nd frequently becomes nn (so andare becomes annà
glbecomes j (which is not actually in the Italian alphabet at all) and is pronounced more or less like a 'y'. so moglie becomes moje and aglio, olio e peperoncino becomes ajo, ojo e peperoncino. (Yum!) Also voja as in nun ciò più voja
The indefinite article is 'n. (Consider 'n'amico mio and, of course, the melodious sarà 'n'a svorta in which we also see the frequent but not universal shift from an l to an r - artro, vorta, quarche...
mo' (see subtitle of this page) means 'now'. E mo' che famo?
fare sega is a specifically Roman expression meaning 'not to go to school' (skive, bunk off, skip school, whatever). Do not confuse this with fare una sega (also farsi una sega), meaning 'to masturbate'.
I don't know if this is representative of a general phenomenon, but in mortacci tua! we see a possessive adjective ending in -a which is actually masculine. (The full expression is apparently i mortacci tua, and indicates even stronger surprise than anvedi oh che roba. You can also say mortacci sua and, I imagine ...vostra and ...loro. I have never heard a first person (singular or plural) form of this expression, though.
(Additional contributions welcome. I'll add links to pages on other Italian dialects if I find out about any.)