http://www.paulschou.com/tools/xlate/
^ That's what I used because it's all in one page. Usually you'll just be using base64 and Hex (base 6)
If it's in pairs of two (usually) with digits and letters between a and f, it's Hex. Put your code in the Hex box, hit decode, and see what comes out.
If it's completely random letters and digits, or has a telltale = sign at the end, it's base64. Put your code in, hit decode, see what comes out.
1's and 0's = binary. (base 2)
Random digits = decimal. (base 10)
A lot of times it will be "double wrapped" or "double bagged"... if more gibberish comes out in the plain text box, figure out what it is and put it in the correct box.
Guilly likes to quadruple bag, and so forth. @_@
Wanna try it out? Decode this (kageyuki [sp?]... don't post the answer!!):
4e 44 63 67 4e 6d 59 67 4e 6d 59 67 4e 6a 51 67 4d 6a 41 67 4e 6d 45 67 4e 6d 59 67 4e 6a 49 67 4d 6a 45 67 4d 6a 41 67 4e 47 55 67 4e 6d 59 67 4e 7a 63 67 4d 6a 41 67 4e 7a 49 67 4e 7a 55 67 4e 6d 55 67 4d 6a 41 67 4e 6a 45 67 4e 6d 4d 67 4e 6d 59 67 4e 6d 55 67 4e 6a 63 67 4d 6a 41 67 4e 6a 45 67 4e 6d 55 67 4e 6a 51 67 4d 6a 41 67 4e 7a 41 67 4e 6d 4d 67 4e 6a 45 67 4e 7a 6b 67 4d 6a 41 67 4e 6a 45 67 4e 6d 4d 67 4e 6d 59 67 4e 6d 55 67 4e 6a 63 67 4d 6a 41 67 4e 7a 63 67 4e 6a 6b 67 4e 7a 51 67 4e 6a 67 67 4d 6a 41 67 4e 54 51 67 4e 6a 45 67 4e 6a 4d 67 4e 6a 67 67 4e 7a 6b 67 4e 6d 59 67 4e 6d 55 67 4d 6d 55 3d