Popular choices are:

• Chem3D Ultra - http://www.cambridgesoft.com/products/

• Cerius^{2 -} http://www.accelrys.com/cerius2/index.html

• SYBYL- http://www.certara.com/products/molmod/sybyl-x

• SPARTAN - http://www.wavefun.com/software/software.html

• CORINA - http://www2.ccc.uni-erlangen.de/software/corina/

It must be emphasised that you should examine the model created by such programs carefully. It is quite possible to build a model that has poor bond-lengths and angles, which actually prevent you from solving the structure. If in doubt always check the Cambridge Structural Database.