If you brought a bike before, I am sure it crossed your mind to put up a bike rack to carry extra baggage. I looked around for options and found best one to be Topeak. Read full review... Main reason I liked Topeak is that they also sell snap on bags. I personally believe its better to have a snap on bag that carry around 3m tape. The Rack goes for about $40 online and probably more around local stores.
The rack came in a plastic bag. I was impressed on how light the rack is. Keep in mind that Topeak racks are also widely used by Law Enforcement Agencies for their bike patrol units. Friend of mine was doubting whether rack will work fine on bicycle with brake disc. He was saying it will be tough to fit the rack over the disc brake.
I have Trek 3900 and had no problems with fitting. I include couple pictures to show you what I had to do to install the rack. Whole process took me about 15 minutes.
There is one screw you need to take out and it may be a little complicated. To take it out, you may need to remove (or at least move around) your rear wheel. But look at the pics.
Out of all racks I've seen, Topeak is definitely the best rack out there with good price and solid quality. I am giving Topeak Bicycle Rack Editor's Choice.