For the price, you can't really beat the Cooler Master Hyper 212 Evo in terms of cooling performance. You can get better air coolers out there, and of course liquid cooling will be the way to go if you've got some serious hardware. But for most of us, this one is an obvious choice. Plus, you can get it for even cheaper than normal today at Newegg, where it's $25 after rebate.
The Cooler Master Hyper 212 Evo is simply the best budget air cooler there is, and it's hard to see any real downsides. It's cheaper than competitors, yet the four copper pipes and the 120mm fan draws the heat up and blasts it away with no problems. It's also fairly compact, so it'll definitely fit in your PC. If you're going to be doing some overclocking, you might want to upgrade, but if you're happy with your specs the way they are, this is the way to go.
It's only been cheaper than this a couple of times before, and it's up at $30 right now on Amazon, so Newegg is offering a good saving after the $10 rebate card. There's also a version that comes with red LEDs, if you're into that sort of thing, but that'll cost you an extra $6 at this point.
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