I agree with the above posts that 18mo might be a little young for computer exposure, but between 3-5, as reading skills start building, using the per-letter TuxType can be a fun game occasionally, and will introduce the computer keyboard to your child. Also, many web apps are there on places like starfall.com, nickjr.com and pbskids.org that have good games for kids that involve reading, math, and comprehension skills
I was surprised to find out that at my daughters Kindergarten last year, their "rediness" assessment was done on a PC. Because my daughter had been exposed to the computer mouse and keyboard earlier, she was able to handle the challenge with less help, and her scores were more accurate to her real skills.
She now also uses the iphone with more skill than either my wife or I, and I'm not sure how I feel about that. ;-)