Iain G0OZS has two moffatig.com/ and g0ozs.org/
Martin G0PJO
Steve M1ACB and his blog
Sam G4DDK
Beacon spot
UK repeaters
ON4KST chat
QRZ callsign look-up
WM7D callsign look-up
Rainscatter points
Find your QRA locator
Coil inductance calculator (and other sums)
Surface Mount Device code book
RF simulation and Filter design:
RFsim99
ELSIE
Filter Designer (Mac)
One of several web-based filter design tools – these by WA4DSY
DOS programs for RF design and analysis from the late Reg Edwards G4FGQ
Propagation and space weather
spaceweather.com
N0NBH propagation panels (as used on this site)
SunSpotWatch.com more graphs than you can shake a stick at.
Stephen McGreevy N6NKS: aurora, VLF propagation phenomena


Got any favourites? Let us know!

Why spend money when you can improvise?


Space for ideas on playing radio on the cheap.

Cheap, easy-to-make Yagis by WA5VJB
Tape Measure Yagi for 2m
Multi-band dipole by GM3VLB

See the Software tab for lots of free software.
Software Defined Radio
HDSDR is a freeware Software Defined Radio (SDR) program for Microsoft Windows 2000/XP/Vista/7.
DSP Radio for MacOS10.6+ Works with the FunCube dongle. By DL2SDR.
cocoaModem is a Mac OS X application which implements modems for some of the Amateur Radio modulation modes using the Mac's 96kHz sound card.
fldigi Digital modem program for Linux, Free-BSD, OS X, Windows XP, NT, W2K, Vista and Win7.
Spectravue A Windows-based Spectral Analysis/Receiver Program primarily used with the SDR-IQ,  SDR-14, SDR-IP, NetSDR,  RF capturing hardware but can also be used to analyse soundcard and wave file inputs.
Spectran is a Windows program to do real time or deferred spectral analysis / waterfall display, in addition to real time audio filtering (band pass, de-noising, band reject and CW peaking) of audio signals, using the PC sound card to digitise the input analog signal, or taking as input a WAV file.
SDR-RADIO.com is a Windows console for SDR receivers and transceivers.
Of course, if you don't have a radio you can always borrow someone else's or go here.

Linking your radio and computer may require a "data mode cable"

Satellites
Heavens Above for all things in space. Set your own site location!
WxtoImg Software to decode APT and WEFAX signals from weather satellites.
AMSAT
Amateur satellite status
POES satellite status Polar Orbiting Environmental Satellites (the NOAA weather staellites)

SlowScanTV
For Windows
EasyPAL is a digital SSTV application designed to run on a PC running MS Windows xP or VISTA.
MMSSTV for analogue Slow Scan TV with Windows (xP, Vista, 7) and soundcard.

For Macs
MultiScan2 Analogue SSTV only
Multiscan3B
Multimode $89 but does a lot more than SSTV
Dream is a software implementation of a Digital Radio Mondiale (DRM) receiver. With Dream, DRM broadcasts can be received with a modified analog receiver (SW, MW, LW) and a PC with a sound card. See here for more information.

For Linux
RXAMADRM Digital SSTV. Uses HB9TLK's HAM DRM standard modes A, B and E (compatible with WINDRM etc.)
QSSTV is a source distribution, so you have to compile and install  it. Instructions here.

Internet
Interace The Internet SSTV program for use with Echolink.
There's a bunch of other SSTV stuff here and here and here but link maintenance is not their strongest suit.
See also G0HWC's SSTV site and KB4YZ

Digital Modes
To see and hear a selection of digital modes and comparisons see here.

Mac software
Mac software for radio 3-page pdf from Scatterpoint June 2011.
FDARS Activity Day April 2011 at Landguard Fort. Pictures by Keith Gaunt.
FDARS Radio Beach Party August 2010 Pictures by Keith Gaunt.