Kids don't know what's boring, and what's not until we teach them. In my opinion it is wrong to assume that kids should only watch programs specifically designed for kids. As such assuming that only cartoons will do is just going to enforce a stereotype that we have invented. Certainly as children get older they want to graduate onto things that they can understand, and this is where kids TV comes into it's own, but at age 18/24 months they will find just the sight of someone talking just as stimulating as a cartoon. I have found that rolling news channels* have a mix of colour and enough different faces/voices to keep my child entertained, also documentaries, animal/wildlife programs, the odd music video too. Quiz shows, daft as they sound are brilliant, they have loads of people, animated shiny/sparkly scoreboards and such, and they'll love to learn to clap and laugh when the audience does etc.
If it has to be cartoons and online, then cBeebies on BBC iPlayer is a good choice. I personally like the recommendation by another user of kids DVDs, you can get stuff from when you were younger that your kids will enjoy as much as you did. A friend bought "Button Moon" for her child and he loves it, plus they are much cheaper than some of the more modern programmes.
- Edit/Addition: This makes it sound like I sit them in front of SkyNews all day, I don't, but if they need 20 minutes downtime to mong then it might as well be something that I don't mind as well