langsandlit:

colloquial, informal
ah sì? – oh yeah? (genuine, sarcastic)
ah sì/già – oh, yeah, I remember
effettivamente / in effetti – you’re right / that’s true (agreeing; resigned if used with suspension points)
eh/eh.. – yeah (annoyed; sad or worried if used with suspension points)
eh sì – guess so (resigned)
e vabbè.. – alright then.. (resigned if used with the suspesion points)
già – yah (not very enthusiastic), yeah (when you want to express approval or agreement to what someone else said, as in già, hai ragione “yeah, you’re right”, but it can also be used to reinforce what you’ve just said, as in già, avrei dovuto dire qualcosa “yeah, I should’ve said something”). Careful: English “so yeah” doesn’t translate to *perciò già or something like that with già.
ma certo! – of course!
ma sì! – of course!
proprio così – that’s right
sì – yes
sì, sì (also written sisì, sisi) – yes, of course (less formal than what it might be in English)
sicuro! – sure!
vabbè (also spelt: va be’ and va beh) – alright (extremely annoyed or sad depending on the intonation/context)

agreeing
d’accordo – all right, it’s a deal
ok – okay
va bene – okay, all right
già – yeah (see above)

formal
assolutamente (sì) – absolutely
certamente – certainly
senz’altro – certainly, definitely
volentieri – with pleasure

inspired by @language2go​‘s post