I like the Prologix IEEE-488 adapters and have three or four here. It's available in two versions. One is USB interface that emulates a standard RS-232 interface and the other is Ethernet.
USB version is around $150 and the Ethernet model is $200. More info at
http://prologix.biz/Let me also relate a story about Prologix's customer service ... One of my adapters was an old Model 4.2 - probably three or four years old - and a few months ago, I tripped over the USB cable. Completely my fault, and the USB connector was ripped off the adapter's PCB. I removed the damaged connector and tried to find a mechanically matching connector, but while I could come close, I couldn't quite match the footprint and mounting holes. I sent Prologix's owner an E-mail, explaining that I had tripped over the USB cable and damaged the connector and asked for the correct part number so I could buy a replacement or if he could supply a replacement. Received a reply E-mail later that day informing me that a replacement would be on its way to me, without charge.
Two days later, a large box arrived from Prologix, and when I opened it up, inside was not a replacement USB connector but rather a new version 6.0 adapter. Needless to say, that degree of customer service is rare in the extreme these days.
Another story about Prologix ... my first adapter was a model 3.0 and I found that it had handshake problems with one particular piece of HP gear, a gain/phase analyzer made by HP's Japanese division. After an exchange of E-mails with the owner of Prologix, and some data dumps, he patched the code and sent the update within a couple days. Problem fixed.