While comparing with a friend opinions about current classical music performers it occurred to me that a major part of the diverging tastes had to do with knowledge. I don't mean knowledge of music but a knowledge of many different kinds of performers. Knowing other options is a major part of appreciation. It's like deciding whether a chef is good when all you've ever tasted were bad or very mediocre chefs. Without comparison, yes it might be good, but admiring that chef for doing something very mediocre is how others see it.
I remember seeing the breathtaking beginning of Spielberg's minority report which explained to me the importance of simply showing you a new kind of possibility (I already mentioned it, and this topic, in Do you gain anything from listening to a concert? ).

What my friend then didn't know was almost any of the classical performers, and without knowing any of them, many current performers seemed good. I wanted then to recommend to everyone to listen to some of these recordings.
Lately, due to the slump in sales EMI needed money and has several "Complete EMI recordings" box sets which are quite incredible and very cheap (though that depends on your country, as the price varies by country). I wanted to mention some of them as I lately have been enjoying them tremendously. Several years ago it was either impossible to get one's hands on these recordings or outrageously expensive.
I'm using classical music as an example as I think it is a strong one but would be very happy to hear recommendations in rock music too (where it is even simpler to get one's hands on the discs).

Everyone I'll mention is great, and amazing, etc. They are all famous but somehow are not so listened to nowadays. 10 Box sets:
 
Piano:
Perhaps the most interesting box set is Cziffra's (which was already mentioned by John Stevenson in Heroic and Poetic music ). The 40 cd set by a pianist who is almost solely known for Liszt (only about 10-15 of the cds were of Liszt) was a revelation. I still found the Liszt somewhat boring but his Beethoven is one of the best I've heard (especially his Appassionata) as is his Chopin and almost anything else.

14 cds of Sviatoslav Richter, many of whom not so well known. Mostly disjoint from his 6 cds in the great pianists of the 20th century series.
7 cds of Dinu Lipatti (though here there wasn't a lot new from old recordings I had). Awhile ago I thought Lipatti was one of the greatest of the Century. I'm not sure if I still think so
15 cds, an old box set of Yves Nat. I had most of them (the complete Beethoven Sonatas and many Schumann, but there are some nice extra cds here).
(Cheap box sets from a year or two ago: 10 cds of Rachmaninoff as pianist and 11 cds of complete Rubinstein Chopin, both in the RCA label).

Violin
There is David Oistrach's wonderful 17 cds set. Oistrach who I knew but not so well until now is probably at the moment my favorite violinist, and certainly the most moving.
A box set of 50 cds of Yehudi Menuhin, who is also one of my preferable violinists lately came out and is obviously great.

Cello:
26 cds + 2 dvds of Mstislav Rostropovich, and another (which I don't yet have but will buy soon)  17 cds of Jacqueline du pré. Both are who I consider the greatest Cellist of the 20th century.

Maria Callas
70 cds, which though were extremely cheap (30 euros) I deliberated whether to buy them, but I highly recommend them. It has 20 operas with the greatest performers, some of them twice, and some recitals. A couple of years ago each opera had cost more than this entire box set. (The librettos are on a CD and not on paper).

Will the prices be even lower next year - maybe. In a couple of years everything will be free. I still wanted these now and not to wait, but in any case you might want to know of their existence. Hopefully Deutsche Gramophon will follow suit and release the complete recordings of their artists soon.