I recently bought one of these:
http://www.conrad.com/ce/en/product/1170561/35-89-cm-disk-drive-Basetech-Refurbished-USB-20?ref=searchDetail
This is a refurbished drive (i.e. they took an old drive, cleaned it, added a USB connector and then put it in an enclosure).
First impression after limited testing is good. My Linux Mint machine even recognised it right away (no drivers or special software needed), which surprised me. It's really cheap as well.
On a side note, from what I understand some of the more 'modern' 3.5" floppy drives only support High Density (HD, 1.44 MB) floppies and can't read Double Density (DD, 720 KB) disks. So this may be something to watch out for (specs aren't always clear about this; my cheapo Basetech is able to read DD disks, BTW).