Hello, I would like to get a Wide Angle Lens for my Sony DSC S75
Camera. I found out there are many such lens in ebay (search term: S75
Wide). My main question is what does 0.5X or 0.45X LENS mean? Is 0.45X
better than 0.5X or vice versa? Would these lens have any fish eye
effect that I do not
want?

Hello vicx-ga,

Thank-you for your question.

The 0.5x or 0.45x is the amount of magnification on the lens.  The
lower the number the wider the picture will be.  If you do a simple
calculation you can see this more easily; 1 divided by the lens
magnification is equal to how much wider the amount of picture you can
get in than normal.

For instance 1/0.45 is approximately 2.2.  Therefore a 0.45x lens will
get 2.2x more in the picture than without the lens.

To answer your question as to which is the best, 0.5x or 0.45x, with
the 0.45x you will be able to get a little bit more in the picture
(but there *may* be a little bit more distortion around the edges). 
In reality, you would probably notice little difference at first
glance between the two lenses.

Judging from the details I can locate about your camera, it is
unlikely that either of these lenses will produce any significant
fisheye effects or distortion around the edges of the image.  When you
look at the evidence of the pictures on many of the eBay auctions this
is confirmed.  For instance, see the following auctions for examples
of pictures taken with and without the 0.45x lens:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=7586525702
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=7585332470

If you have any further questions on this subject please do not
hesitate to ask for clarification.

Further reading:
http://en.wikipedia.org/wiki/Fisheye_lens
http://en.wikipedia.org/wiki/Wide-angle_lens
http://en.wikipedia.org/wiki/Angle_of_view
http://www.zeta.org.au/~andrewa/ajaa31.htm

Thanks palitoy-ga a lot for your quick and detailed explanation!