SOURCE: TheRegister

BT has started handing out Broadband Accelerators to customers prepared to stump up ?1.20 in postage, and promises an average speed increase of half a megabit.
BT customers interested in a free boost to their broadband can apply on-line for a free Broadband Accelerator, assuming they're prepared to pay for postage, have the right kind of BT socket, and aren't already getting the promised bandwidth from BT.

The device is actually BT's I-Plate, and it has been around since last year, retailing for under a tenner. It blocks the bell wire that's prone to picking up in-home interference, as well as providing some filtering on other lines. Anyone with extensions around the house should get one, as the improvement to ADSL connections can be dramatic. This is why BT is handing them out with such enthusiasm.
The bell wire is supposed to provide a signal to make extensions ring, but modern phones don't use it - it doesn't even pass through the ADSL microfilters that should be fitted to every socket. So it just acts as one big antenna picking up rogue signals from around the house - such as those generated by florescent lighting - and channelling them into the ADSL line to interfere with the data connection. Simply cutting the bell wire also works, as well explained by Jarviser, though the filters in the I-Plate may improve things even more.
Recognise this socket: then you could benefit from a free I-Plate

The I-Plate won't help those of us who've been pushing our speed by any means available (currently topping 288Kb/sec, thanks for asking), but if you've got any extensions and the right kind of BT socket, then it seems worth a try for less than the cost of a half.