+ Firefox  (full support for MathML) (users must install the MathML fonts).   This browser renders MathML extremely well and acts in a courteous way if users hit such pages without having the required fonts installed (in which case, they simply are unable to see the characters).

+ IE   (support for MathML in the form of the MathPlayer plug-in).  

+ Amaya  (full support for MathML).  This browser renders MathML very well, but may not be fully stable yet. Read the small section on Amaya in the introduction of Arun Ram's MathML for mathematics research articles - he offers a few constructive comments on this browser. This is an intriguing browser, and well worth a try.

+ Opera  (ver. 9.25, Feb 2008) (no support for MathML).  While this browser can not render MathML, it does not hang or do other horrible things when you hit a page containing it. It is, in fact, a very courteous, well-behaved browser in all respects.

+ Safari   (the Nightly Build of Safari based on Webkit now has capabilities for rendering MathML.) The report I received from Paul Windey (thank you Paul) describes the development work on this browser as follows (June 2011):
"Not quite where Firefox 5 is but most things are quite readable. Accent placements is deficient and fancy things like Euler Fraktur fonts produce errors. Otherwise well worth a try."
(Bravo Safari! Marvellous news that you're taking the initiative to implement MathML rendering capabilities for your browser.)

Other XML/XHTML/HTML rendering engines capable of rendering MathML

+ PrinceXML

(Do you know of a browser or rendering engine capable of rendering MathML?  Please drop me a line, I'll be happy to list it.)