Well, I feel like a late-comer to this party! Everyone and their brother seems to have discovered these before me.
For those other latecomers (and I was gratified to be able to explain them to a colleague at work who is usually as geeky as I am about pens and paper...) these are erasable rollerball pens. It's the heat of friction (hence the name) that deletes the ink and apparently the heat from a hairdrier also works (making them useful for craft purposes too).
Whilst I am far too wedded to my fountain pen to write letters etc with a Frixion pen, they really are fantastic to keep my diary up to date and neat. I don't particularly like having to write my appointments in pencil because that's a bit too faint, but then, my schedule can turn on a sixpence and if I write things in ink I am then left with either a load of crossings out, or a page whose thickness is doubled from the application of tippex.
Then I heard about these pens. I was a bit sceptical at first having bought erasable pens in the past and either taken them back to the shop or binned them because they didn't erase. But having heard good things about these Frixion pens I tried them and I'm converted. You will have guessed from the rest of my blogs that I'm no fan of cheap paper and I think the pens might struggle on anything without a decent surface, but for my purposes they are great. They're perfect for my appointments diary and also for making notes on the run (I love my filofaxes too much to risk tucking a fountain pen into them - not that they really fit in the pen loops anyway!).
Just as long as I don't leave my diary on the radiator, huh?