enclosures are the safest bet as they protect against dust, shock (ie from knocking over to SOME degree) and many will come with fans to keep the drive cool.
that being said, i use a usb to ide adapater similar to what xtacydima posted above. i try not to leave the drive on for long periods of time though...the heat thing concerns me.
the adapter is so flexible...you can switch out 3 or 4 drives within seconds looking for a file or whatever you need.
either way, i'd make sure that whether you use an adapter or an external enclosure you ensure that the device has its own power supply. I haven't personally seen it happen, but I've read a lot of ocmplaints on messageboards where the USB 2.0 connection wasn't supplying enough power to the drive itself causing erratic behavior.