I was more than satisfied with an EnGenius EUB-362 USB WiFi adaptor for a couple of years after using a few others including a Linksys USB adaptor, a PCMCIA card device and one other USB device; can't recall the brand. Then Engenius came out with their EUB9603H USB adaptor which is significantly more sensitive than the EUB-362. Both are not very expensive. A feature of a USB adaptor that makes it more versatile than a PCI card device is that with an active USB extention cable you can move the antenna to a location where it works better if you have a marginal strength signal where you want to locate your PC or laptop. I have used mine during travel with two 15' active extension cables in series.
I recall one time when the motel where we stayed had $10 per day WiFi but with an EUB-362 I was able to pick up an unkeyed signal from another motel that I could not see that was located on the other side of a motel next to ours. It was almost a half block away.
The EUB9603H has provision to connect to two USB ports but I use just the single main connection and it works very well for me.
C Crane has some WiFi products that appear to be impressive but I have no experience with them.