I thought I would kill two birds with one stone by taking a picture of all my favorite running gear and talking about why/how I use each piece.
Race Ready LD Fitness shorts w/mesh pockets - I love these short and wear them on almost every run (I have 2 pairs). Even when it is cold, I'll often wear pants over or under them so I can still have the storage space. The snug fit prevents chafing, and the mesh pockets in the back are perfect to carry everything I need during a long run without bouncing, including my cell phone and nutrition.
Pearl Izumi Seamless Arm Warmers - This is perhaps the best investment I ever made. I love wearing these when it is cool out instead of a long sleeve shirt. Because they fit snugly, they do a better job at keeping me warm and dry by wicking away sweat. When it warms up, I can simply roll them down to my wrists or tuck them into my pockets.
Bondi Bands - These wicking headbands help keep my hair in place and sweat off my eyes, and they are cute too! You often can't read the cute sayings because I usually wear a hat over it. I wear these anytime I work out, whether it be yoga or cycling or anything else.
Garmin Forerunner 305 - Obviously I use this to track my distance, time, pace, etc. I LOVE data! :) I rarely run without the watch, but sometimes ignore the display and analyze the data when I get home instead.
Body Glide - I use this everywhere, especially for long runs or hot days, to avoid chafing.
Nutrition - Last year I ate only sports beans for my half marathons and long runs because I couldn't stand gels, but I quickly grew tired of those. I finally started eating Gu gels this year, and I love how compact they are so I can carry more for the marathon without weighing me down. I usually also bring along a pack of clif shot blocks and mini luna bar to mix things up so I don't get sick of eating gels.
Amphipod RunLite Snapflask 4 Hydration Belt - When I run with the Galloway group, they have water stops so I rarely need to carry my own hydration. However, there are times when we start too early or I'll go run on my own, so I'll wear this. It can hold up to 4 8-oz bottles, but I usually just carry 2. If I position it just right and snug enough, it doesn't bounce around and I barely notice it. I much prefer the extra weight around the waist instead of carrying a bottle on my hand. I also own a Amphipod Handheld 12 oz Water Bottle that I used for shorter runs in the summer, but I try to avoid carrying anything when I run if I can help it.
Spi Belt - I use this as a race belt (using safety pins to attach the bib to it) more than for storage, although it comes in handy to carry keys, ID, and money when I don't want to carry a purse.
Pearl Izumi Jacket (not pictured) - I bought this at the Pearl Izumi outlet for cycling, but it has come in handy for running in the cold more often. It is a light weight wind breaker, but has zippers under the arms to help ventilate when it starts to get warm or you can remove them altogether to turn it into a vest. It is also water resistant.
I am sure I am forgetting some essential piece of gear, but these are the basics. Of course, I also use a good sports bra and tech shirts. What's on your list of essential gear? Do you have any questions about what gear to use for a specific situation? Let me know in the comments!