XKCD Nerd Sniping.
Math Pages How to solve the problem in 'Nerd Sniping'.
Group Theory This is something I never learnt much about, when I studied physics it was not part of the syllabus. For a beginner this course looks more understandable than most.
Azimuth The John Baez website, this is the place for all the 'cool math' stuff.

The Feynman Lectures on Physics
Much of what I know about physics was learnt from the Feynman Lectures, and these are now available free online. They were written around 1963, so some of the material is out of date, but on the plus side many errors in the original books have now been corrected.
Not Even Wrong The traditional view that theory needs to be confirmable by experiment appears to have been abandoned by some of the string theory crowd, here a leading opponent of this outlook, Peter Woit, argues his case.
Susskind Lectures Something a bit more conventional. For those who enjoy watching paint dry. Recommended.
Physics Stackexchange Questions and answers.
Britney Spears' Guide to Semiconductor Physics This is actually a serious physics site.
QED - The Strange Theory of Light and Matter By Richard Feynman. An explanation of the quantum theory of light and matter aimed at a 'nontechnical' audience, from 1982. If you prefer to listen to the man himself the lectures can be found on YouTube in 8 parts.
The Meaning of Einstein's Equation by John Baez and Emory Bunn. An introduction to General relativity at an easily understandable level.

Free Stuff
Project Gutenberg Free books. But mostly old, with expired copyright.
Spotify Free music. The free version has occasional interuptions between tracks by 30sec adverts. I originally used Grooveshark, but this is sadly deceased. Spotify has a useful 'Discover' option in the 'Browse' section, and this suggests artists to try based on those you already listened to.
Major Geeks Free software. Not all entirely free, lots of shareware.
Learning Piano Online A good teacher is the first preference, but for those with lots of free time and not much money these free online resources are worth a try.
MOOC. (Massive Open Online Course). For many subjects online learning looks like becoming 'the next big thing', and includes courses run by some of the top universities. Here in the UK university tuition fees have been increased from 3000 to 9000 per year, which may turn out to have unintended consequences. Why spend a fortune on an uncertain quality of teaching when you can learn from some of the best lecturers online at any convenient time and no cost. Provided meaningful qualifications can be included this could be the future of advanced education.

Audio and Electronics
Wireless World is available online at American Radio History. Many old issues going back to 1913 are included, but there are still a few missing. I started reading the magazine around 1968, but gave up soon after it changed its name to Electronics World in 1996.
diyAudio Audio discussions. Lots of opinions mixed with lots of interesting and occasionally useful information.
Hydrogen Audio A more restricted discussion forum, sound quality opinions not supported by double-blind tests are strongly discouraged; stick to the facts! More reliable, but perhaps less entertaining as a result.
Elliot Sound Products. A wide range of audio related topics are covered on the Rod Elliot website, including many practical designs.
Head-Fi Don't annoy the neighbours, get some headphones.
HeadRoom More headphone test results.
Electronics Tutorials I learnt about circuit design many years ago when silicon transistors had just become easily and cheaply available, so there was relatively little published material. Now there is so much available information and it is more difficult to know where to start. This site is one place to start, but watch out, there are a few errors. If something looks wrong maybe it is. Spotting mistakes may be a good way to learn, look at Fig.4-B in part 1 of the transistor tutorial and try to figure out what is wrong. An analysis showing how the resistor values should be calculated can be found later under 'An Audio Amp' in part 8.
Op Amps For Everyone. A free book from Texas Instruments. As suggested by the title this is mostly about op-amps and their uses, but it starts with a 'Review of Circuit Theory' suitable for beginners. I particularly like section 2.6 about 'Superposition' which is maybe not as well known as Kirchoff or Thevenin, but can be just as useful.
The Semiconductor Museum has lots of information about the early history of transistors, but does seem to concentrate mostly on USA devices. Here in Europe we were more likely to be using the OC71 for audio and OC171 for radio circuits, both made by Mullard.
How to make really good homemade PCBs. Exactly what it says.
AIM-Spice 5.6. I am still using version 4.1a. This is not necessarily the best version of Spice to use for audio circuit design, but it does everything I need well enough, and is free. The free version has a limit of 150 nodes and 30 transistors. LTspice is sometimes recommended by audio enthusiasts, but I never tried it myself.

Higgs Boson Blues by Nick Cave. The sound quality is not great.
Fannin Street 'Anywhere I Lay My Head', is Scarlett Johansson singing Tom Waits songs, this track includes David Bowie as a backing singer.
Waltzing Matilda, (It's really called 'Tom Traubert's Blues') from 1977, that was the first time I saw and heard Tom Waits, live on the 'Old Grey Whistle Test' a late night BBC music program. I heard an interview where he was asked how anyone with a voice like his could have chosen singing as an occupation, and he said it had been a choice between that and a career in air-conditioning and refrigeration.
Van der Graaf Generator are also likely to be either loved or hated. Never as popular as some of us thought they should be. This is 'Darkness' from around 1970. If you like this try their album 'Godbluff', one of my own favourites. They were formed in 1967, the same year as the death of Robert Van de Graaf, the American physicist who invented the Van de Graaf generator. One of their early songs is called 'What Ever Would Robert Have Said.'
Wreckless Eric and Amy Rigby sing his song 'I'd Go The Whole Wide World'. This has a special meaning for me, I met my wife 8,000 miles away. (New UK immigration laws, 2013, now make this impossible for many people. We might hope this is just a temporary aberration, but there are already stories of married couples finding they are now not allowed to live together in either of their countries. The financial 'justifications' are ludicrous, the real point is evidently to compete for the anti-immigrant vote.)
K D Lang Crying - From the 'Unplugged' series, my favourite version of this Roy Orbison song.
Raglan Road by Van Morrison and The Chieftains. Try also the Sinead O'Connor version.
Bandcamp, this page has tracks by my son, who composes electronic synthesised music. It isn't the sort of music I usually listen to, so it's hard for me to judge, but I quite like the second track. Have a listen and see what you think.
The tracks at Bandcamp can also be downloaded in various formats for a fee, but you can contribute as much or little as you like, anything from zero upwards. Bandcamp keep 15% and pass the rest on to the artists.

Food and Drink
BBC Food Food and drink are covered in these BBC pages.
Sangria Recipes I don't much like most wines, but I do love sangria. It was mentioned in a Lou Reed song, Perfect Day, which is why I first tried it, but not in the park. You may prefer the original version of the song here.