Dataone Usage Finder Ver. 1.5:
Only the monthly total of the bandwidth used is provided on the BSNL
website. There is no provision for knowing the daily total. Also there
is no demarcation between free time usage and paid time usage. These
are the very figures a user would be most interested in.
This program uses IE5 or higher, to fetch pages from the BSNL site, and
after parsing out the required records, presents them in a manner which
is more helpful. This data can also be saved as html files.

http://duf.sourceforge.net/

Note: As this application uses data from the BSNL website to show your
usage details, it CANNOT be used to track if BSNL is billing erronously.

Update:

I myself using this software now & its really great. The best feature I think is it generates an approx bill according to dataone plan you are using. It has presets for all plans to calculate your approx bill. It also gives daily hours of usage & transfer, you can check for any date. Everything will be presented in bar graphs which is very good.