Well, I didn't vote, but thought I'd post, as this is something I commonly did when the boys were younger, but now that they are 10 & 12, 'aint gonna happen!
When we went anywhere there were large crowds, I dressed DH and the boys in exactly the same shirt. I always went for a bright color, like lime green or orange (which looked really good on my fair skinned dark hair guys!) and when we went to DL or WDW, it was the same Disney themed shirt, I still tried for a color that was "different" than the rest. I went even further, and typically had them in the same type pants (jeans, jean shorts, khaki shorts, etc.) and socks. Whether it was useful or not, it always gave me comfort that if the kids got separated from us, it would be easier for them to describe Daddy to someone (he's got "this" outfit on), easier to "spot" in a crowd, and we'd be able to tell security/police "he's dressed 'just like this'". The boys actually enjoyed this themselves until my older one was 7 or 8. Then I could get away with the same shirt, but different colors for awhile. Now, even though they both wear the exact same size and share all of their shirts, they don't want to look even close to the same when they're out together.
Zoe and I tend to wear the same colors when we go out and about, but nothing "planned". I've looked for matching and haven't been able to find alot that will work. Though, now she's bigger (size 7/8) so I may have better luck.
Grandma made matching dresses for Zoe and I for my nieces baptism. This is the first we've been really "matchy matchy", and even with that, there are embellishments that hers has that mine doesn't.