The EFF's Panopticlick service can show you the fingerprint of your browser, and evaluate how unique you are on the internet. For privacy concerned people, the less unique you are, the best it is. Notice that the panopticlick's test is biased, because only privacy concerned (and maybe a little paranoid) people use it, so that this service don't exactly give a measure of how unique your browser's fingerprint is amongst all browsers, but more precisely how unique your browser is amongst browsers used by privacy concerned people.
For Firefox users, here are some addons smoothing the indentifying infos your browser leaves on the internet: , so diminishing your browser's fingerprint:
1) NoScript fully activated (but be aware that as soon as you allow scripts on a site, your fingerprint can sharply raise)
2) TorButton
3) JondoFox (it already contains NoScript).
4) FireGloves Almost as efficient than Torbutton + Noscript, but when used you can be unable to sign in some web services susch as webmail.
5) Blender (less radical, but less effective than Noscript or FireGloves).
6) Blend In (I just discovered this addon and don't yet have used it ; As Blender it seems to be less radical but less effective than NS and FireGloves).
Edit: An interesting paper about fingerprinting:
https://panopticlick.eff.org/browser-uniqueness.pdf
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