1.) A pen.
This is pretty simple, but absolutely necessary. Nothing is worse than needing to write something down and not having anything to write with. A simple Bic pen would do, but if you want to show that you belong in the professional world, a metallic pen always works wonders and last a lot longer. I carry around a Cross Classic pen that I got as a gift many years ago.
2.) A small notepad.
Now that you have something to write WITH, its important to have something to write ON. Nothing says 'I'm still a teenager' than writing down phone numbers or important information on your hand. Obviously you don't want anything too big or bulky, but having something to write down info on that you can access at any time is invaluable. I picked up a 3-pack of ruled mini-notebooks by Moleskine from Target and they fit great in a back pocket. It also has pages that are detachable, perfect for handing off a note to a friend or co-worker.
3.) A pocketknife.
Having a small pocketknife is a great all-around tool for most daily situations. Rather than having to look for your scissors to open a package, find a screwdriver to tighten a screw, or search for a bottle opener for that mircobrew you just bought, pulling out a pocketknife can fix all of these situations in a flash and will make you the go-to person in times of need. And don't worry, you don't need the super-megadelux Swiss Army knife with a gazillion different attachments, one with a blade and a bottle opener will solve most issues at any time. I have an older Swiss Army Cadet, its really thin and light, and has anything I need.
(Note: I think it goes without saying that when traveling by air, this item should be left at home.)
4.) A THIN wallet.
Obviously every man carries a wallet, but how many times have you seen a guy pull our what appears to be some amalgamation of leather, bills, plastic and paper receipts that is the size of a frozen burrito? Don't be that guy. I GUARANTEE you that you don't need the Taco Bell receipt from lunch two weeks ago. At any time, the only thing a guy should need would be an ID, one MAYBE two credit/debit cards, and some walking around money. A collection this small requires only a money clip or card case, and should slip effortlessly into a back pocket sans bulge. I carry a generic metal money clip, but anything like this will do. I really would like to invest in one of these one day...
5.) A flash drive.
With everything being in electronic form, carrying around a flash drive can be very handy. Nowadays, you can get a cheap flash drive with 8GB for under $10. Store some important documents like your updated resume, references, or a list of important contacts, because you never know when you might need them. I carry around a Lexar 8GB and have yet to fill it up even halfway.
These are things I have found useful and essential on a daily basis. Is there something you think I left out of the list? Feel free to comment!