* Astronomy

Although I'm a purest for a clean light transmission through a telescope i'm also a big fan of filters. Filters can have a large and real effect on your observations. 

Mars appears as a tiny red/orange disk even in large telescopes so filter colour, light transmission, telescope magnification, and atmospheric seeing are fairly important factors when trying to observe the planet.
Generally, warm or reddish filters will reveal the best surface detail; colder bluish filters are usually best to reveal clouds and fogs and identifying dust storms ).
#80A Blue  (30% transmission).  
#82A Light Blue  (73% transmission).  

Greenish filters (#58, #64) will darken red/blue features and enhance polar regions; and may also highlight clouds and lighter/white areas.
#56 Light Green  (53% transmission).  
#58 Green  (24% transmission).  

Magenta, an often overlooked filter, will enhance red/blue features and darken the green features.

Expand (108kb, 1024 x 655) Credit: NASA/JPL-Caltech/Cornell/Arizona State Univ.	This image mosaic (thumbnails are blurred to give a general colour range) was taken by the Opportunity rover in mid-January 2012 and shows a Martian vista from a location informally named "Greeley Haven."

A #11 Yellow-Green filter (78% transmission) may enhance contrast and reveal detail of martian plains. The filter may darken the bluish planetary features. The high light transmission filter can be used even in smaller telescopes (again, to observe any real detail on Mars, you do require a large telescope).
#56 Light Green  (53% transmission).  
#58 Green  (24% transmission). 

For the martian polar caps a green, red or magenta filter may also increase the contrast. At opposition of the planet, the polar caps should be visible with a 90mm telescope.

Reddish filters will reveal the best surface detail. 
#21 Orange (46%  transmission). 
#23A Light Red (25% transmission). 
#25A Red (14% transmission).

FLD filters
The pale magenta coloured Fluorescent Filter-Daylight, or FL-D, FLD, are a general correction filter or balance filter, for use under many types of fluorescent, and non-continuous spectrum light sources. It provides correct colour in photography, by removing the yellow-green or blue cast caused by fluorescent bulbs with daylight-balanced film (5500K).
The similar FL-W is designed for shooting under fluorescent lighting with Tungsten-balanced film (3200K/2900K).

Astronomy wise, these filters are excellent for bringing out detail on Mars (or Jupiter). Both FL-D, FL-W are rated as '1 stop' and are suitable for small telescopes. These will bring out martian polar detail.

The red FLB filters are also used to correct colour (designed for removing green cast caused by fluorescent bulbs when shooting with tungsten film) and can also be used to observe Mars. This filter, as to be expected, will give high contrast to surface features.

The FLD filters are relatively cheap to buy from any photography store. 
The filters come in various sizes, the smallest seems to be 30mm, (larger versions can be cut down to size) and can be refitted into empty 1.25" (28.5mm) filter cells, or just held above the eyepiece.

Minus-green filters

The Wratten#32 magenta filter or 'minus-green' filter is similar to the FLD filter.

The Wratten #85 amber filter (1.5 filter factor, 2/3 F-stop) is a colour warming filter that takes an outdoor scene lit by sunlight (which has a colour temperature around 5500 kelvins) and makes it appear to be lit by tungsten incandescent bulbs around 3400 K. This allows an indoor balanced film to be used to photograph outdoors. These filters were used in Super 8 movie cameras that were designed to use Tungsten film.

Both of these warm filters may also bring out martian planetary detail. 

Colour Chart

Specialist filters
As many users have found, the Baader Contrast Booster is a great filter for Mars. It will enhance surface features and the Southern Polar Cap.  Price ~£40

Orion Mars Filter 1.25"
The custom-designed Mars Observation filter enhances contrast of virtually all surface features. Dielectric coatings transmit violet/blue (615nm) wavelengths where detail is rich, while blocking overbearing yellow/green (515nm-615nm) wavelengths. Polar icecaps appear more sharply defined, and differences in landscape shading are nicely resolved. Polar hazes and cloud cover become visible. The filter's high transmission makes it effective even in small telescopes. Price $114.00
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Tele Vue Bandmate Mars filters
Tele Vue have two contrast enhancing filters specifically designed for viewing Mars (also useful on Jupiter); the Mars Type-A and the Mars Type-B. 
Traditionally, standard coloured filters have been used to bring out detail on the planet's surface. The main drawback is the artificial colour imposed on all planetary features.

TeleVue Bandmate Type A Mars Filter - 1.25"
The Mars Type-A is a dual-band filter with different dielectric coatings on each side of the substrate. The dual band nature of the filter permits green and red to pass through while rejecting all other visible colours. The effect is to enhance detail on the Martian surface while preserving the natural colour of the polar caps and rest of the surface. Price $129.99
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The Bandmate Mars Type-B filter is a high efficiency, single band dielectric coating that achieves penetrating views of the Martian surface. The surface appearance is similar to that of the popular #21 filter. However, the Type-B features a broadband anti-reflection coating on one side and filter coating on the other that makes it far more efficient.
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Mars with Skywatcher Evostar 90

Credit torbjornsholmqvist

This video of Mars was filmed at approximately 21.30 UTC in the evening of February 9, 2012, with a Skywatcher 90 mm refractor and a Logitech C250 modified webcam together with an IR-blocking filter with a 2X Barlow lens in front of the star diagonal roughly giving 3X on the focal length of the scope. The picture is mirror reversed so the dark area close to the northern polar cap is Mare Acidalium and down to the left Mare Erythraeum can be seen.