I use a cheap WiFi dongle on my Raspberry Pi 2 that I bought from eBay for $2. The chip inside that dongle is the Realtek 8188eu and you can find a great article with updated drivers for this chip here.
Its extremely low cost is justified if you realize how cheaply it’ s been manufactured. The USB connector is actually the circuit board itself and the connector pads are copper tracks on the board. My Raspberry Pi disconnects frequently from my WiFi network even though I have disabled power management for the WiFi dongle. It reconnects either if I unplug and plug it back, or if I just keep pressing it down with my finger.
It was pretty unusable and I disassemble it to try a simple fix. I tinned the copper pads using my soldering iron. That fix made the copper pads a little thicker which is noticable when I insert the dongle in the USB socket. Also it improved the conductivity between the socket and the plug, and prevents them from oxidation. The dongle seems to work. It doesn’ t disconnect so frequently but it hasn’ t been fixed completely. I will buy a couple more for backup and hope they will work better.