Depends on your amiga, the a1200 and a600 have the pcmcia slot, there are pcmcia to cf card adapters they are very cheap 10 usd price range.. Most people with an a1200/600 would use that i guess
There are sd card readers in the 30 usd region that plug into the ide port as well. Although i guess they are not hotplugable. (Don't have one)
On an interesting side note i saw discussion on the gotek thread asking if its possible to use the gotek as a cheap usb stick reader. Of course the limitation here is its max read speed will be the floppy transfer rate which i think is 500kb/s.