$20 SDR hardware

Thanks to Osmocom cheap USB stick DVB-T tuner can be changed into SDR (Software Defined Radio) hardware.

Inside EzTV666/668 dongle you can find RTL2832U DVB-T COFDM demodulator with high-speed USB device and E4000 multi band RF tuner (64MHz to 1700MHz input frequency range, from FM radio to DVB-H). The RTL2832U can be placed into a mode where it simply forwards the unprocessed raw baseband samples (up to 2.8 MS/s 8-bit I+Q) via high-speed USB into the PC, where they are routed into GNU Radio making excellent value SDR.
No hardware modifications are needed, so USB stick can still work as DVB-T or consumer band radio receiver.
Other popular homemade SDR solution is to use sound card as a A/D converter. Receiver kit for the 40 m (7 MHz) band can be bought for $15 from http://www.amqrp.org.
Overview of SDR applications for Linux (usbsoftrock, quisk DttSP + sdr-shell included): http://kb8ojh.net/sdr/linuxsdr.html.
For Windows users: SDR# (SDR-Sharp) versions 244+ (dev) have added support for RTL based SDRs supported by ExtIO.

Check also page dedicated to RTL-SDR: http://www.rtlsdr.com/.

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.