Due to some miracle, I won a Radeon R9 290X some time ago. Performs great.

However, like many people on the Internet, I have found the heat and noise this GPU generates to be insane! Not even gaming, I see the card go up to 94C. Turning up the fan to 80-90% brings the temperature down significantly, but the noise is like a hair dryer.

After some reading, I stumbled upon Fixing The Radeon R9 290 With Arctic's Accelero Xtreme III at Tom's Hardware. The Arctic Accelero Xtreme III seems like a very good solution for both the heat and noise. Based on the article, I see that the configuration is just a little to long to fit in my case. If I remember right, a Fractal Design R3 has enough room to fit a 300mm card. The R9 290x with the Accelero Xtreme III comes out to be 320mm.

I was wondering if anyone else had any recommendations on an after market cooler that would perform well, be compatible with the R9 290X, and also be under 300mm.

Prolimatech MK-26 is the best GPU cooler you can buy. It fits nicely without adding to the overall length of the card. With two very low RPM 120mm fans the card will be near inaudible and very cool. This comes at a price: the MK-26 is 85$ +two fans you're in the neiborhood of 100$.
According to this Youtube video the Arctic Accelero Twin Turbo II works, in the comments someone writes works for gaming, but not mining.
There's "the mod" or the Arctic Accelero Hybrid, using an Asetek OEM closed waterloop on the graphics card. This solution should have the lowest teperatures, but it introduces pump noise and the fan(s) are not as quiet as the ones you can use on the Prolimatech MK-26.
I bought Thermalright Spitfire and Shaman coolers on ebay because they can cope with any graphics card and they are no longer sold elsewhere. You need patience and maybe some luck for this.

I can vouch for "the mod". Yes, it does introduce water pump noise, and unfortunately in my experience the pump noise has a lot of sample variance. The asetek (round type) coolers are more convenient for "the mod" because they do not cover the GPU mounting holes, however they're louder than CoolIT and have more variance. The CoolIt pumps (square, mainly used by Corsair) are quieter, but are far less convenient to mount on a GPU. Since DWOOD no longer makes the brackets for square pumps, I would have to say the only choice for the mod is round Asetek AIO's. The pump noise, depending on how bad it is, may mean that your overall system noise is going to be higher at idle, but at full load it is going to be the quietest solution you can buy. It only takes a single 1200RPM fan to cool 290X to 50-65 degrees depending on the quality of mount and the case, that is far far quieter than any other aftermarket fan. And if can stack two fans on each side of the radiator or get 240mm radiator or if you're willing to live with 70 degrees you can go even lower than 1200RPM.

I agree with boost, MK-26 is by far the best aftermarket cooler, but you do need a case that can fit it, and you will load with a very heavy cooler the PCIe slot. Im pretty happy with my Artic Accelero III Extreme, the only thing bad is the way nvidia design the pwm of the card doesnt work with the artic fans very well, so i have it running at 7v where is very quiet and still have very good temps.

The manufacturer aftermarket cards are starting to show up, the windforce seems like a decent alternative, Gigabyte Radeon R9-290X WindForce 3X OC review, but the card still goes over 80s, if the MK-26 works would be a good bet imo to tame such a hot card.

I went custom water cooling for my R9 290 after using a dwood bracket and AIO cooler for my HD6950, but if the idea of "The Mod" interests you NZXT is producing the G10 bracket which is a commercial kit to adapt any Asetek cooler to a GPU.

As I see it, the main benefit of going this route [water, whether AIO or custom] as opposed to a heatsink is to get the weight off the card; most cases have the cards hanging parallel with the ground, which puts a lot of stress of the PCIE socket unless you can support the card by, for example, tying the power connector corner of the card to the top of the case with a fine cable.

Thanks for the suggestions. I did not think to consider the weight/stress on the PCIe slot. I'll have to remember to take that into consideration as I shop around.

If you wish to see the results of R9 290X + MK-26, check Redvineal post#7950, idles at 35C loads at 66C, pretty good for a card that on stock goes over 90C.