A blanket depending on the hospital. All the hospitals Ive ever stayed at have been freezing and none of them had extra blankets to use. Luckily my DH brought me my favorite blanket from home to cuddle with when I was cold.
Pictures of your other children if you have any.
Snacks! Hospital food SUCKS I always get sick after eating it. So, whenever I know Ill be there more than a couple of hours I bring a bag of snacks. One time I didn't (ambulance ride) I was there 6 days and DH kept forgetting to bring me food so I was so hungry by the end.
Cell phone and charger. Charger for laptop/IPod/anything else electronics. Digital camera and charger just in case you go into labor in the middle of the night. That way your partner doesn't have to think about something else while trying to get to you.
slippers or extra thick socks. Preferably slippers if you are going to be able to get up and walk around at all, don't want to slip and fall on top of everything else.
shoes to wear in the shower (maybe its just me but communal showers are yucky to me, I always wear shoes when I'm in them)
Any meds. Might sound odd but I have asthma (luckily i haven't had an attack since I was pregnant with DD1) and I didn't realize they would want me to have the inhaler while in labor (just slipped my mind), it took 7 hours for them to get the prescription approved, have the pharmacy fill it, send someone down to get it and bring it back up. I was only in labor at the hospital for 4 hours so it didn't make it before end of labor.
Anything that you use or might want to use when you get stressed. If you have a favorite music/pillow/lotion/soap/whatever that you use just when you are upset/stressed then bring it. You will probably need it.
Razors if you shave, I lucked out the one time I was in the hospital for non-baby related illness that it was a military hospital so they snagged a razor from one of the male hygiene kits for me.
Journal, writing paper, extra pens/pencils. Crossword or other puzzle book.
If you knit or do needle point/other crafty work, then bring that. You will have a LOT of time to kill and movies/music/books are going to get boring after a while.
If you can fit your best friend in your bag