Here is a Leica Lens post, first of all I am no expert here just had to do my research when I joined the Leica rout.
I'll make this mainly about Leica R lenses that are discontinued because Leica doesn't make Reflex cameras any more. One good thing about this lenses is their focal flange distance of 47 mm, therefore it can be adapted to Nikon, pentax, canon, or any other mount. So if lenses are the real investment because they last longer than cameras then it makes some sense to buy lenses that can fit any camera system. (Maybe Nikon makes a dslr that shoots raw). This is no 30 euro lens thread but if you know what and where to look then you could find a lot of sub 200 euro lenses.
What I find good about this lenses: excellent color reproduction long focus barrel rotation and they practically don't breath when focusing.

So lets start with the Leica nomenclature for f stops
(There are plenty more but this are the most common)
f/stop Name
f/0.95, f/1 and f/1.2 NOCTILUX
f/1.4 SUMMILUX
f/2 SUMMICRON
f/2.5 SUMMARIT
f/2.8 ELMARIT
f/3.5, f/3.8 and f/4 ELMAR, SUPER-ELMAR

So when you see a Leica lens you must see the name and the serial number.
Serial numbers are very important because they tell you the production year and the model of the lens.

This French site tells you in which year a lens was made
http://summilux.net/numeros/

Then here you can see which model the lens is
http://www.overgaard.dk/leica_lens_compendium.html
(scroll down to: “R-lenses for R3, R4, R6, R6.2, R7, R8, R9”)

Here you can see which lenses work with canon 5D without hitting the mirror
http://www.pebbleplace.com/Personal/Leica_db.html

This is good to know because then you can see when a lens is over priced or not.
For example a 3900000 Summicron 90 costs 2900 euros while a 2800000 costs 500 and a 90 Elmarit 2100000 is 250 euros. Of course prices vary depending on the condition of the lens. I bought a 90 Elmarit with dust inside for 170 euros.

So the idea is to find a good leicas really cheap, there are old leica lenses that don't work on new leica R cameras. Those sometimes are labeled as SL/SL2 or Leicaflex (those are the first slr cameras leica made.) With patience you can find a 1st model Summicron 50 for 125-200 euros; the Elmarit 135 for 170-250 euros.

Now an intresting story is the leica 24mm. Leica didn't have good wide angle lenses in the 70 and no zooms. They really liked minolta's stuff so the 24mm is actually a minolta lens, the same happens with the 16mm fisheye and the 35-70 f3,5 zoom. 70-210mmf4, 75-200 f4.5 and 80-200 f4.5 all minolta made.
With the m43 those minolta lenses are a huge deal I think way better than the fd's. The 58mm f1.2 is one of the best f1,2 ever made, and the zooms can be found dirt cheap and has the same optics as the leica.(the good versions)