Shortening codes

Gentle introduction to Code Golfing in Perl: Perl Golf Step by Step Christian Helbling and Remo Gisi — coding for fun (pdf)

Other tasks for shortening codes:
EXPR2 (79 chars - C by Mitch Schwartz)
NOP (38 chars - Perl by hbm)
GCD (72 chars - Ruby by Mitch Schwartz)
BRAINF_K (128 chars - Perl by Lukas Mai and Jander)
MAGIC2 (32493 chars - C++ 4.3.2 by Mathieu Chartier)
CARL (73 chars - Perl by Mitch Schwartz)
BFWRITE (117 chars - Brainfuck by Mitch Schwartz)
SELF (142 chars - C by Shinichiro Hamaji)
DEC_BIN (123 chars - C by Linghui Liu)
XMAS (109 chars - Pyth 2.7 by hallvabo)
DIE_PIP (69 chars - C by [Retired] Blue.Mary)
WNAME (32 chars - Ruby by Mitch Schwartz)
LIB (97 chars - Brainfuck by Mitch Schwartz and Sandeep Garhwar)
NUMWORD (202 chars - Pyth 2.7 by Mitch Schwartz)
MODULUS_2 (232 chars - Brainfuck by Mitch Schwartz)
BALLS (118 chars - Pyth 3.2.3 by Mitch Schwartz and Linghui Liu)
PLDGTS (310 chars - Perl by Mitch Schwartz)
PRINTF (58 chars - C by Mitch Schwartz)
FINDPROB (92 chars - Perl by Mitch Schwartz)
EASYMATC (96 chars - Ruby by Mitch Schwartz)
GCD4 (113 chars - C by Linghui Liu)
AGSCHALL (169 chars - C by Mitch Schwartz)
FIBFUNCH (168 chars - C by Mitch Schwartz and Linghui Liu)
EPROBLEM (97 chars - Ruby by Mitch Schwartz)
BMS1988 (371 chars - Pyth 2.7 by hallvabo)
CSPATH (99 chars - Perl by Mitch Schwartz)
SUMUP (34 chars - AWK by Mitch Schwartz, Mostafa 36a2, Linghui Liu, and Tjandra Satria Gunawan)
M5TILE (52 chars - Ruby by Mitch Schwartz)

Tasks in Vietnamese (but most of them are easy to understand):
adding two numbers (6 chars - AWK)
comparing two numbers (18 chars - Perl)
sum of XORs (22 chars - AWK)
scoring (17 chars - Perl)
printing VNOI x times where x is the length of the code (14 chars - Ruby)
counting users (29 chars - Perl and Bash)
evaluating (7 chars - PHP)

Useful sites:
Code Golf in Perl/PHP/Python/Ruby
Programming Puzzles and Code Golf on Stack Exchange
Code Golfing tips
Anarchy Golf
PHP Golf
Perl Monks
Regex Golf
Project Euler code golf in K
Vim Golf
Questions on Stack Overflow tagged "code-golf"
Ruby Golf
Instruction golfing - game