Wireless Network Adapter
I built this as an upgrade over my 2011 Macbook Pro as a station for doing school work, using the internet, and a little bit of light gaming. Matlab, Microsoft Office, Multisim, and general computing all seem to be pretty noticeably faster than my Macbook, and it's definitely a lot more comfortable to use a nice mechanical keyboard, wireless mouse, and 22" monitor than it is to sit at my laptop. The timing of this build also happened to coincide with the Steam sale, and the AMD A10 with integrated graphics performs pretty well at the highest settings on the cheap old <$5 games that are within my budget such as Bioshock and TF2.
My budget for this build was strictly $300 and the keyboard, monitor, mouse, and OS I already had. I would have made it too if my Comcast modem/router combo could actually be moved to a different room, but the house is too old for there to be more than one cable connection. In the end, I had to buy a PCI-E wireless card which pushed me over $300, but close enough that I'm still satisfied.
Most of the parts were purchased secondhand locally from other college students selling PC parts or used on Amazon. I didn't haggle too much on them, but the prices still beat sales on Newegg, Tigerdirect, and Amazon due to tax and shipping. The AMD A10-5800K was acquired for a very nice $85 used off Amazon and doesn't appear to have any problems. Buying used parts was surprisingly painless and ended up saving me a lot of money considering the budget.