Wow, that thing is small!
And there's an ad banner at the top of this forum for it.
I have a Holux GPSlim 236. While it's not the smallest GPS receiver on the market, it does use the highly available Nokia BL-5C and 6C batteries, which are handy.
i'm using the Holux 240 on my N73 Music Edition together with Tom Tom Navigator 6 and it's just perfect. The Holux 240 is really light and small and you can even wear it like a necklace