Bumbo

When our friend (a professional photographer) took pictures of our baby at 4 months, she introduced us to the Bumbo, a baby seat made out of a plastic yet sort of squishy type of styrofoam. It's shaped in such a way that holds the baby up in a seated position. We got away with "the green chair" for as long as we could, which served a similar purpose, except in more of a lying down position. The green chair worked great on hard floors because he could kick his legs and rock himself. The Bumbo, or "the blue chair" as we now call it, is a great thing to have to sit him somewhere safe while we do something quick, like take off a coat or shoes. But we didn't buy one because we didn't feel like spending $40 on something that we would only use for a few months. Quite literally, the very day that we had our baby sitting up on his own for about2-3 minutes at a time (before flopping backward or sideways), our friends with a newborn loaned us their Bumbo. It has been great, but we are estimating a solid 2-3 weeks of use before he has outgrown it. At 7 1/2 months, he thrusts his butt up to the point that he is very close to getting himself out of the chair, and I definitely don't feel safe leaving him unattended in it!
You might see in the photo that it comes with a tray, but we never had the chance to use it since he was already pretty big for it.
In conclusion, definitely get one if it is free from a friend.