[I'll say] or [I tell you] {interj.}, {informal} I agree with […]

[I'll tell you what] or [tell you what] {informal} Here is […]

[I'm all right, Jack] A remark, often directed at another, indicating […]

[I'm telling you] {informal} It is important to listen to what […]

[I.O.U.] {adj. phr.} I owe you, abbreviated; a promissory note. * […]

[IANAL] "I am not a lawyer"; usually used before a non-lawyer […]

[IANARS] "I am not a rocket scientist"

[IC] "[I see]", also "[in character]" in [MMORPG] or [MUD] settings.

[ice] See: [BLOOD RUNS COLD] or [BLOOD TURNS TO ICE], [BREAK […]

[iceberg] See: [COOL AS AN ICEBERG].