I'd like to improve my skill as a reverse engineer. And thus I am looking for a location to find these.
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Here is a just closed reverse engineering challenge that was posted by Halvar Flake: http://addxorrol.blogspot.kr/2013/01/encouraging-female-reverse-engineers.html
The winner and a link to her very detailed and well written report is linked on this page: http://addxorrol.blogspot.kr/2013/03/congratulations-marion.html
This is much more difficult than most crackme's that I have encountered and is an example of a complex and obfuscated piece of Windows malware.
Also you should check http://crackmes.de/ They have a huge list that you can sort by difficulty.
open an account if you don't already have at forum.tuts4you.com, then browse to http://forum.tuts4you.com/forum/37-crackmes-unpackmes-keygenmes/ I believe that you will find many that you will like.
Along with the crackmes and tuts4u sites some conferences have reverse engineering challenges. They are typically harder than the average crackmes. AthCon recently put up a challenge. Here's a link to the zip if you are interested. I haven't personally done the challenge myself, so I don't know the skill level.