AOKP, CM9, and I believe MIUI as well, have an excellent spell-checker. If you misspell something, it's underlined red (sometimes a different color based on the theme you're using) and you can correct it later just by clicking on the word. I haven't seen this in any of the other ROM's I've tried, and as crazy as it sounds, it's become a selling point for me. I don't usually make too many mistakes, but when I do I prefer to correct it after finishing writing a text/email before thumbing through for any words I either need to correct or add to the dictionary.
Is it possible to add this feature into other ROM's, or is it something that wouldn't really be able to be tossed into another ROM?
I've checked, and it doesn't seem to matter what Email app, texting app, or keyboard you're using, the spell-checker is always there and working excellently.
Any feedback is greatly appreciated.