A pack of condoms?

disclaimer: i haven't any children.

I like the idea of a gift certificate. I would put it in a card with a small, very nice picture frame for the baby's room.

[Upon a re-read of your post, maybe a child's book for the 2 year-old and gift certificate for the parents. I'm sure the older child will really appreciate feeling special, too.]

We had friends that, for both of the kids, sent some gourmet treats right after I got out of the hospital. That was awesome because while I was drowning in baby supplies, what I didn't have enough of was fudge-covered Oreos.

We also got some cute matching Big Sister/Little Brother t-shirts, which made the girl feel more included.

Or you could make up a basket of things that you can never have too many of...like cloth diapers (for burping and vomit wiping!), onesies (they crap through them with alarming frequency), cute baby socks (easily lost). The first kid has probably chewed through all the baby teethers and board books in the house, too.

And then there's my favorite baby product...RashStick. It's diaper rash stuff that goes on like deodorant so your hands don't get all goopy. I mention this not so much because it would make a good present, but because you should totally lay in a supply for the Podelette.

Fresh stuffed animals that haven't been chewed on yet are always nice, I think (not a parent, just an observation).

I've also given new books (especially board books, see above: chewing).

Oh! You know what else was awesome, that I bought for myself but wish someone had given me? This Baby's First Year Calendar. I was terrible at keeping a baby book for my first kid, and knew it just wasn't going to happen for #2. But sticking a sticker on the right day, I can do. (I also got the matching Big Sister book for the girl, but it sounds like your friends' kids are still too young for that.)

Anything that's "personalized" for the 2nd kid, not necessarily with like a name engraved or anything (though that could be nice), but something that really belongs solely to the kid. I feel bad for our little one sometimes...everything is a hand-me-down from his big brother. "Daddy, where did I get this robot?" "Uh, that's your brother's trash." This is not very specific advice and I know it's pretty hard to personalize a gift for an unborn person, but just something to keep in mind.

A few months of maid service.

"a" maid service... I'm still on my first cup of coffee this morning.

Photo-related stuff. Speaking as someone who was a fourth child (or seventh, if you count my foster sisters) and who has *zero* baby pictures of herself.

Something I do that's always well-received is to buy a small-ish longstitch kit with a babby-appropriate picture, make it up (small-ish and longstitch so it's 3-4 hours to make, I do it while watching TV) and either frame it for hanging on the wall or make it into a cushion cover. Handmade, personal and inexpensive, but of course I have no idea if such a thing would be your cup of tea!

I like the baby's first year calendar idea.

A card with a movie gift certificate and restaurant certificate for first date night out without baby is good. Offer to babysit. (Just kidding.)

Get them something both kids can enjoy, like a height chart.

Books. They don't have to be baby board books. Any children's book(s) is a great present. You can never have enough. Even more special is books you enjoyed as a child.

When my ex-husband was born (he was brother number three out of three), brother number one asked if they could send back brother number two now. Nothing like brotherly love.

Gifts for a second child?

a third child. he'll need someone of his own to ignore.

RashStick! How did I even survive?! Damn I wish that had been around.

Small toy for the first kidlet is good, and anything really small and disposable - onesies, bibs, washcloths.

Maid service or babysitting credits or something like it, too. Mompop will be chasing older kid around while trying to keep younger kid from dissolving the first few months, and that gets pretty tiring.