Silicon Labs makes many excellent 8051 based MCUs. All the MCUs in the EFM8UB line have a native USB and are pre-programmed with a USB HID bootloader. EFM8 Flash is a small GUI tool to flash the MCU from a binary file. The program uses the wxwidgets toolkit for a native look in all supported operating systems and hidapi for cross platform compatibility.
EFM8 Flash is a simple implementation of the protocol described in section 7 of the Silicon Labs application note AN945 "EFM8 Factory Bootloader User's Guide". The utility is very simple to use. Use the browse button to select the file to flash. Make sure the file is in binary format. To convert it from the Intel Hex format output by Silicon Labs Simplicity Studio use the srec_cat command from the S-record package.
EFM8 can currently flash only firmware less than 15K on the EFM8UB1. It will detect hotplug of the MCU at any time. Press the flash button to write the firmware on the MCU.
The archive also contains a command line version of the utility useful for scripting.