For the last 2 years it has been my iPod..... I have one of the old school 5gig models I can't imagine all those train / bus journeys without chilling out to some music.
A close second would be my Canon S45, I've taken sooooo many pics on it and only really started becoming interested in photography once I purchased it. In some ways I've gone from digital to film with my investment in current SLR kit. No doubt I will be going back to digital once I save up the $o$ for a digi slr.