Here you will find links to several useful programs and websites. My favourites are printed in bold.

Structure viewers

CrystalMaker, a very user-friendly commercial crystal structure manipulation/viewing suite. (Mac, Win, commercial)
Vesta, an outstanding free crystal structure viewer (Mac, Win, Unix, Linux, free)
Diamond, another popular commercial crystal structure suite. (Win, commercial)

Single crystal structure solution

XPREP, manipulation and analysis of .hkl files, absorption correction, etc. (MS-DOS, commercial)
SHELX-97, structure solution and refinement. (Mac, Win, Unix, Linux, free for academic use)
PLATON, structure analysis, transformation, twinning etc. (Mac, Win, Unix, Linux, free for academic use)
TextMate, a powerful text editor for Mac. (Mac, Shareware)
SHELXL.bundle, different tools for single crystal structure solution/refinement/analysis for TextMate. (Mac, freeware)
WinGX, a GUI for several crystallographic programs. (Win, free for academic use)

Powder diffractometry

TOPAS Academic, Rietveld refinement and powder structure solution suite (Win, commercial)
jEdit, a versatile platform independent text editor (Mac, Win, Unix, Linux, freeware)
jEdit - TOPAS Mac Bundle, modified version of John Evans’ jEdit TOPAS Bundle for MacOS X and Parallels Desktop (Mac, free for academic use)
GSAS & EXPGUI, Rietveld refinement suite (Mac, Win, Unix, Linux, free for academic use)
CMPR, analysis of powder data (Mac, Win, Unix, Linux, free for academic use)
DICVOL, indexing of powder patterns (Mac version on request, Win, Unix, Linux, free for academic use)

General sites

Fraunhofer IFAM Dresden, website of my current employer.
LMU München, AK Prof. Johrendt, website of my former workgroup at LMU München.
CCP14, a very complete collection of links to free crystallographic programs.