An (incomplete) list of EF lenses worth getting and lens buying guide

Introduction/disclaimer: I'm making this list and guide to help those less knowledgeable about lenses have an easier time deciding what to get. I've compiled information for various websites and have gathered a list of lenses that have gotten the best reviews/perform well/test well. The list definitely is open to critique and feel free to suggest your own lenses/reviews! I didn't include kit lenses in the lineup because, well, most people will be looking to upgrade FROM those to newer lenses.

Here are some useful links to start stuff off:
--Good links for buying used and/or new lenses:
http://ebay.com
http://keh.com
http://adorama.com (used: http://www.adorama.com/catalog.tpl?op=category&cat1=Used)
http://bhphotovideo.com (used: http://www.bhphotovideo.com/c/browse/Used-Equipment/ci/2870/N/4294247188)
--Compare lenses/test their sharpness here: http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx
--Read reviews here: http://www.the-digital-picture.com/Reviews/Canon-Lens-Reviews.aspx / http://www.fredmiranda.com/reviews/

Now lens time...
Canon Brand Lenses:

-PRIMES-

Sub $500 (prices based off of B&H new price):

Canon EF 24 2.8 -$360
Canon EF 28 1.8 - $480
*Canon EF 50 1.8 II - $100
*Canon EF 50 1.4 USM - $380
*Canon EF 85 1.8 USM - $390

Sub $1000

Canon EF 15 2.8 Fisheye - $635
*Canon EF 100 2.8L IS USM Macro - $970
Canon EF 135 2.0L USM - $1000
Canon EF 200 2.8L II USM - $780

Price no object

Canon EF 14 2.8L II USM - $2220
*Canon EF 24 1.4L II USM - $1650
Canon EF 35 1.4L USM - $1390
Canon EF 50 1.2L USM - $1550
*Canon EF 85 1.2L II USM - $2090
Canon EF 300 4L IS USM -$1350

-ZOOMS-

Wides
Canon EF-S 10-22 3.5-4.5 USM - $800
Canon EF-S 15-85 3.5-5.6 IS USM - $760
Canon EF 16-35 2.8L USM - $1580
*Canon EF 17-40 4L USM - $780

Standards
Canon EF-S 17-55 2.8 IS USM - $1100
Canon EF 24-70 2.8L USM - $1300
*Canon EF 24-105 4L IS USM - $1070

Zooms
*Canon EF 70-200 4L USM - $660
*Canon EF 70-200 4L IS USM - $1250
*Canon EF 70-200 2.8L USM - $1350
*Canon EF 70-200 2.8L IS USM - $1550
*Canon EF 70-200 2.8L IS II USM - $2500
Canon EF 100-400 4.5-5.6L IS USM -$1590

Third Party Lenses

-PRIMES-

Sigma EF 15 2.8 Fisheye -$610
*Sigma EF-S 30 1.4 -$440
Sigma EF 50 1.4 -$500
Sigma EF 85 1.4 -$900
Tamron EF 90 2.8 -$450
Sigma 180 3.5 -$690

-ZOOMS-

*Tokina EF-S 11-16 2.8 -$600
*Tamron EF-S 17-50 2.8 -$460
Tamron EF-S 17-50 2.8 VC -$650
Sigma EF 18-50 2.8-4.5 -$200
*Tamron EF 28-75 2.8 -$500
Sigma EF 70-200 2.8 -$800

Old/Used Lenses

Old lenses are great. For their very cheap price you can get a full manual lens that out performs many of the 2000 dollar lenses for 1/10 the price. Most old/used lenses require an adapter to be used on canon slrs. These can be purchased relatively cheap on ebay or amazon. There are a TON of gems out there, some discovered, some still relatively unknown, that all will blow you away. Be aware that not all lenses do work though, be sure to double check the mount type and find the appropriate adapter. Note: do NOT buy adapters with glass elements in them, stay away from these. They degrade the quality of the picture and

Mounts that can be adapted to fit Canon dslrs:

m42 screw mount - very common
contax c/y mount
nikon mount - also popular
olympus OM mount
leica mount
pentax mount

Some, but not all, old recommended lenses. (prices vary so not listed)

Nikon 50 1.2
Nikon 85 1.4
Nikon 50 1.4
*Pentax SMC 50 1.4
*Helios 44-2
Olympus Zuiko 28 2.8
Carl Zeiss C/Y 50 1,7
Pentax Super-Takumar 50 1.4

(Note: stared lenses are popular and recommended on this board time and time again)
(Note: more used/old lenses will be added to this list by members in posts below)
(Note: EF-S lenses can ONLY be used on cameras with a crop sensor (APS-C size) and cannot be used on a full frame camera)

Lens Buying Guide

First, start with your budget. How much money do you have to spend?

Second, what kind of lens do you need? What will you be using it for?
Wide (sub 28mm), standard(28mm-85mm), telephoto(85mm+), all around zoom, tele zoom, wide zoom.

Do you want something that is fast (good in low light)?
If yes, you probably want to go with a prime because they offer a faster f stop. If no, you could probably go with a zoom.

Do you want a prime or a zoom?
zooms have more flexibility, primes have a longer focus throw and are usually faster than zooms

What lenses do you already have?

Do you mind using a full manual lens?
If you don't, buying an older lens will save you money while still getting a superb piece of glass. Some people want a lens that works with Canon's autofocus system for photography.

Answering all these questions will help narrow down what kind of lens you really need. Check the lists above, read some reviews, compare quality, and you should be able to find a lens that matches your needs perfectly! Happy hunting!