Crucial's MX100/MX200/MX300 series of drives seem to be much more flexible about downscaling from 6Gb SATA III to 3Gb SATA II than competing brands like Samsung. The Crucials are also available in Micron branding (BX100/BX200/BX300). I've used several of these in 2009-2010 MacBooks, with no hangups. OWC recently introduced a low-priced line (OWC Neptune 6G; 240GB for $80, 480GB for $128) that has also worked successfully, although their higher-priced SSDs are 3Gb/6Gb-specific.
A point for the SSD nerds: Neither the Samsung nor the OWC drives can be modified with trim utilities such as Trim Enabler or Disk Sensei. Both manufacturers use Sandforce controllers, which have their own cleanup mechanism.
Another suggestion I got from OWC tech support: If you're installing an SSD as an upgrade in a laptop from 2011 or before, replace the HD ribbon cable (attaching the drive to the logic board) with a brand-spanking new one. The old cable has gotten banged around in the years you've used it, and SSDs are more finicky about a clean data connection. I have no empirical evidence for the OWC guy's claim, but I've installed 14 SSDs in 2011 or older laptops; the ones where I installed new cables worked, and the ones where I didn't install them didn't work until I swapped the cables. YMMV.