usually the large 50mm fan or larger fan models are the best in a single 80mm or dual 50-60mm fan U-mount works good. but you'll have to bend off tabs. some models like the hardcano also work well. make this a consideration for >250GB disks or 7200+RPM drives. prevents drive stiction from overheating. I am pretty sure this is what happened to someone's 300GB drive.

5.25" metal HD case. Internal ball bearing Fan. <18 dBA (Low Speed). Air Flow: 53.2~100 CFM.

U shape. single 80mm fan. very quiet. doesn't come with appropriate screws (not long enough). cools HD's about 10 degrees C, case about 5 degrees C according to reviewers. you have to bend the little tabs/brackets in your HD bay that hold up the drive for this to fit. 57% were very pleased.

metal 5.25" HD cooler that uses a 40mm ball-bearing fan (easily moddable to hold 2 fans). very loud fan (one fan blade is longer than the others - cheap mfg problem). Because of fan problem, it could vibrate your hard drive and put bad blocks on it. off-center-weight cd's (usually some older ones) or cd's with off-center labels can cause HD vibration-data-loss too. However, "easily modded for dual replacement quieter fans." 25.6 dBA.