Well, Google Transliteration Bookmarklets is just what you were looking for.
Type just like you would to write the bangla word in English usually and see it turn into bangla. What's best, you don't need to install a software or download something ridiculously lousy that shows at start up everytime.
How does one use it?
Just drag and drop the text below to your bookmark toolbar (you can enable it by going to View>Toolbar>Bookmark Toolbar in Firefox or Ctrl+B in Chrome):
Type in Bangla
It will look like this:
Now when you want to type that comment in Bangla on a blogpost, or on that Gmail Chat window, just click on this bookmarklet on your bookmark toolbar and wait for a text that says:
Transliteration is enabled. Please click on a textbox to start using it.
You can now toggle between English and Bangla by using Ctrl+G. Oh and you can click on a word to see other word suggestions.
On that note a Happy Birthday to Google!