Phillips GoGear Spark 4GB. Very very tiny, surprisingly good user interface, and quite good sound quality. Fits in the tiny pouch my Sennheiser in-ear phones came in alongside the headphones. Very impressed with this thing for $69 from DSE on sale. The only real downside is I had to convert my AAC to MP3 for it as it doesn’t support many formats.
I picked it up as my Nexus One sounds absolutely dreadful through my in-ear phones, but really great through my huge Sennheiser HDsomethingsomethings. I’m suspecting some impedance mismatch. At first I was thinking of getting some better quality large headphones for the Nexus One (my Sennheiser HDs I picked up very cheaply for gaming and have a really cheap and nasty cable) then realised that a micro MP3 player would be both cheaper and easier to carry around than new headphones.