Bluetooth to serial modules have been around almost forever. Just connect power, ground, RXD, TXD, maybe a config pin and you have a trivial way to transmit data wirelessly from an embedded system to a PC or Android device.
A very common example is the HC-05 module (below):
I remember it being the first hobby-affordable module. It has two chips – the (former) Cambridge Silicon Radio SoC and external flash to hold the firmware. The main problem I face with this module (and all others) is that they don’t have a single manufacturer, so you never exactly know what you are going to get when listing “HC-05” in a BOM or buying from Aliexpress. There are many datasheets and the modules can differ in some non-obvious ways (eg. different AT commands, different default baud etc.), so I decided to shop around for new hardware.
I found the JDY-30 module on Aliexpress (top picture). As always there are plenty of mislabeled devices, so always look at the picture. It is cheaper than HC-05, has a single more modern chip with ARM9 core (possibly consuming less power than CSR) and is footprint compatible with the HC-05. Perfect… until you actually try to send arbitrary data.
The modules I got identified as +JDY-30-V2.7, Bluetooth V3.0. Reception (from Bluetooth to UART) worked just fine, however when I started sending data nothing happened. I eventually found out that the module is waiting for a newline to send the data, so it is limited to textual data. Maybe one day the firmware will be upgraded to send data in a binary-transparent, but for now I will avoid JDY-30 modules.