Every now and then, I go into major re-arrangement mode with regards to stuff in my computer bag. Everything is needed but I am always amazed in what I carry with me whenever I go out on a job:

Of course, if I could, I would put one of my guitars in there so that I can have a 'jam session' while waiting for those pesky blue bars to complete their journey from left to right. Alas, my Brenthaven shoulder bag is not big enough to accommodate any of my guitars:

Now, y'all need to appreciate the reasons for all that stuff; especially the cables. It all comes down to getting a result with a client on the first visit.
The cables are a mixture of firewire, USB and ethernet. All with their various respective types and lengths. At least once a week I will need to leave a cable with a client because either the cat chewed on the printer cable or they lost their firewire cable or their network won't work because an ethernet cable is faulty. Of course, I never know what the fault is until I get there.
The mobile hard disks - one of them has three partitions (Snow Leopard, Leopard and Tiger); used for starting up from on Intel based Macs. Another one has the same partition scheme but for use on PPC based Macs - both also have additional Utility software installed. On the third mobile HD I have software images stored. The final one is used for backing up a client's Mac prior to working on it.
The CD case mainly contains OS software disks (10.2, 10.3, 10.4, 10.5, 10.6) and Utility disks (DiskWarrior, Drive Genius etc). There were empty sleeves left over so I put various other CDs in there. Again, all this comes down to my personal experience where a client, for example, cannot find their own OS CD/DVD.
The tool case. Well, sorry but no self respecting engineer goes out on a job without a set of screwdrivers.
The remaining stuff is just a mixture of lil' ol' bits. USB hub, 16-in-1 card reader, dictaphone, Edirol R-09HR Wave/MP3 recorder, pens, moleskin notebooks, elgato eyetv diversity.... and most important - a small hip flask with Glenfiddich single malt in it for those moments when one can 'murder' a drink.
The end result? I can guarantee as much as I possibly can that when I go out on a job, I will get a result the first time without inconveniencing further my client by having to return again because a cable, or a CD or whatever is required to continue the job. This is an important element of my business plan - my company's 'vision statement' if you wish.
It looks like a lot of stuff; and it is. But it all fits very snuggly and nicely in my bag and really does not weigh that much.