Alternative firmware for USBasp

USBasp is a well-known simple USB AVR programmer. It was released as open source project in 2005 by Thomas Fischl and quickly gained popularity. If you have spare ATmega8 or ATmega88 it is easy to build one on your own, although MCU has to programmed first, so you may face chicken-egg problem.
For any USBasp user I would greatly recommend looking at alternative firmware set: USB AVR Lab. This software increases USBasp capabilities allowing to switch its firmware almost instantly and reusing hardware. It creates kind of "software defined" lab tool (hence the name) that can function as:

• AVR programmer with either original USBasp firmware, AVRISPmkII, STK500v2 or JTAGICEmkII emulation,
• JTAG/OpenOCD interface (unfortunately: still alpha),
• generic USB to digital I/O interface,
• digital sniffer supporting SPI, UART and I2C protocols,
• most impressive in my opinion: low-speed digital oscilloscope (230 kSps).

If you have more than one USBasp hardware you can even combine it into multichannel oscilloscope, although I think that lack of synchronization between channels might make a problem in some situations.