EDIT #2: Apparently, you can't donate everywhere in the US either. Here's a map I found for donation centers that may be helpful.
I posted in an entry a few days ago about donating plasma, and wanted to share with everyone in case they're curious about how to pick up some spare cash.
The first time you go to donate, it's a rather long process (at least it was for me). They show you a video and talk a lot about the safety procedures and when you can and cannot donate.
On a 'regular' trip to donate, they do three things - check your weight (if you're under 110 or so, you can't donate), check your iron count (with a prick to your finger - if it's too low, you can't donate) and they go over a series of questions (have you gotten a tattoo in the past 12 months, traveled to Africa, etc). If you pass those 'checks', you go back to the waiting room with your paperwork until you're called. The place I went to had a large variety of relatively current magazines, but a lot of people would bring their own reading or study material.
Once you get called, you go to the donation area and get in a recliner-like chair with your paperwork. They ask you which arm you want (I always did my left arm so I wouldn't have scars on both arms) and they set you up to go. The needle sticks don't hurt too much, but there are times when they need to move the needle around, which can be uncomfortable. They also give you a squeeze toy to pump with the hand of your donating arm, to help speed up the process. Once you've filled your bag (roughly an hour), they give you a saline solution (through the same needle) to replace the fluid you just donated. Takes about 5 minutes or so - it will feel cool coming into your body since the saline is at room temp. It doesn't hurt at all, but it is a weird feeling. After that, they bandage you up and you get paid. The place I go always gives cash, which is nice. You can also set up your next appointment at that time. Sometimes for new donors, they have specials (I've seen first time people get $30). You can only donate twice a week, but it's great if you want to get paid to read or listen to headphones and squeeze your hand every few seconds.
I donated a lot in college and it was nice to have some spending money as a result! Advice before donating would be to eat a regular meal beforehand and drink lots of water - it makes the donating a lot easier! If your iron count is too low, you can't donate either, so make sure you're eating right (or taking your vitamins, you little Hulksters!)
We've got a local plasma center that only does donations, but otherwise a look in your local phone book or a call to a local hospital should help you find a place near you.
I haven't donated for a few years, so the procedures might have changed, but let me know if you have any questions.