On the advice of a friend I started looking into the Cokin Creative System of filters. It's an interesting system, because 1 can fit almost all type of lenses. Cokin makes this available in 4 different flavors for different type of cameras. The P-System will fit most DSLRs with lenses ranging from 48mm up to 82mm. These measurements are for the outer lens diameter. What's so nice about this system is that first it is very inexpensive, compared to the screw-on type filters, and second it is very versatile. For different types lenses all you need is the adaptor ring, which here in Canada retails for about $15.00. On the adaptor ring goes a plastic holder, that's one size fits all. You insert the filters, which are square 84mm sheets of organic glass, into the holder. Now comes the fun part. There are a lot of different filters, and I mean a lot. The holder can accommodate up to 3 different filters, which gives a lot more combinations of creativity, and the filters are about $25.00 for most types. Here are a couple of examples I took early in the morning, with a gradual fog filter.
Canon 5D, ISO 400, 1/60 sec, f/5.6, 24mm
Canon 5D, ISO 400, 1/320 sec, f/5.6, 24mm
You can have a lot of fun with these, and they won't hurt the wallet.