Hi, there--congratulations on your and your girlfriend's pregnancy, first of all. :)
I know it can be a little nervewracking to be anticipating that trip to the hospital, but a little planning and preparation can go a long way toward making you feel better about it.
The hospital takes care of all the real "necessities" that your girlfriend will need when she does go into labor, but there are a number of things she can pack to make herself (and you!) more comfortable. They include:
Clothes: for the time that she's in labor, she'll be in a hospital gown only, so that's not a big deal. Afterwards, though, she'll want some loose, comfy clothes for the the two-three days that she's in the hospital. She'll be tempted to pack non-maternity clothes, but make sure she realizes that the weight doesn't come off that quickly, and she'll really need some roomy pants and big tops to be comfy. The hospital will provide some mesh undies, but she may wish to bring some maternity underwear--it's more comfortable.
Slippers or lounge shoes for when she does get up and moving about.
Toiletries, such as shampoo, body wash, body lotion, a little light make up (unless she really doesn't mind that visitors see her without her "face" on--that's the last thing on a lot of women's minds), hand cream-whatever stuff like that she would normally take with her if she were going to be away from home for a couple of days. She will have a shower and bathroom right there in her room, and will want to use them.
She may want to bring her own pads, even though the hospital will provide its own huge pads.
If she'll be nursing, a nursing bra and pads.
Cell phone with contacts already programmed into it, or a list of contacts for someone to make calls after baby is born. (That's always nice, anyway--mom and dad will want to rest, relax, and spend time with baby.)
Camera, baby book, book or magazines to read and keep her mind occupied.
Her own comfy pillow--I had a small, narrow pillow that was especially comfortable tucked in against my side during the last few months of my pregnancy and after I had a c-section.
Some chapstick, because your lips tend to dry out in those hospital rooms.
Diaper bag for baby--don't pack it too full, but consider a special outfit for newborn photo and a few sleepers/onesies and receiving/swaddling blankets that have already been washed in your detergent and fabric softener. I have found with my three children that the receiving blankets, especially, at the hospital, were a little rough on the skin.
Also, at our hospital, we were allowed to unpack each day and take home any formula and diapers from the infant crib, so the diaper bag is handy to stash those items in.
If you and your girlfriend have a birthplan, be sure to bring that with you.
Also--any soothing, relaxing music to play during labor is good, especially on an ipod.