There are many tools for writing and posting comments in Malayalam. Some of them can be availed from the links shown below. They allow you to enter text in Malayalam using a Roman keyboard. You can type a word the way it sounds using English alphabet (see the table) and these tools will convert the word to Malayalam. The converted content will always be in Unicode.
aana = ആന
ithaa iviTe vare = ഇതാ ഇവിടെ വരെ
indulEkha = ഇന്ദുലേഖ
The best Malayalam typing tool may be Key Magic, which works perfectly with Notepad, MS Word, comment boxes of blogs / websites, various browaers etc. Download link and more info can be found HERE.
Indic Language Input Tool from Microsoft helps you enter Indian language text easily into any application in Microsoft Windows or on any page on the Web. The primary input mechanism is transliteration. Additionally, it provides a visual keyboard to assist with editing words that do not transliterate properly. It has both a web version and a desktop version.
Another helpful tool in writing Malayalam is Typeit, developed by Ramachandran. It can be downloaded from his blog Mottusuchi.