b/c: Because
w/e: Whatever
thx: Thanks
kk: Ok
lolz: same as LOL (laugh out loud)
ttyl: Talk to you later
cya: See ya
brb: Be right back
idc: i don't care
idk: i don't know
gratz: Congratulations
plz: please
tbc: to be continued
TBA: to be announced (often seen on school schedules for class locations that haven't been decided yet)
lmao: laughing my a** off
lmfao: laughing my f***** a** off
roflmao: rolling on the floor laughing my a** off (there are many more of these adding more vulgarities... typically, the longer the abbreviation, the harder someone is laughing)
ez: easy
tl;dr: Too long; didn't read
tl;dw: Too long; didn't watch
inb4: in before (used when making an obvious pun before someone else has the chance to make the pun; hence "before")
b4: before
l8tr: later
... I'll post more if I can think of them...