The Chem3D ActiveX/Plugin can be used to view many file formats.  Here are some examples:

C3XML (Chem3D native file format)

ChemDraw CDX file

MDL MOL file

Sybyl MOL2 file

PDB (Protein Data Bank)

Macromolecular CIF

Here is a complete list of the file formats that can be loaded into the control:

File Format Name File Extensions
Chem3D XML .C3XML
Chem3D 8.0 .C3D; .C3T
Chem3D 3.2 .C3D
Alchemy .ALC; .MOL
Cart Coords 1 .CC1
Cart Coords 2 .CC2
CCDB .CCD; .DAT
ChemDraw .CDX; .CHM
ChemDraw XML .CDXML
Chemical Markup Language .cml; .xml
Conn Table .CT; .CON
Gamess Input .INP
Gaussian Input .GJC; .GJF
Gaussian Checkpoint .FCHK; .FCH
Gaussian Cube .CUB
Int Coords .INT
ISIS Sketch .SKC
ISIS Transportable Graphics .TGF
MacroModel .MCM; .DAT; .OUT
MDL MolFile .MOL
mmCIF .CIF
MSI ChemNote .MSM
MOPAC Input .MOP; .DAT; .MPC; .ZMT
MOPAC Graph .GPT
Protein Data Bank .PDB; .ENT
SDFile .SDF
SMDFile .SMD
SYBYL .SML
SYBYL2 .MOL2; .SM2; .ML2
SMILES .SMI