I know that I am more than a little late, but I still intend to go over last year's best mashups. I also intend to do the (extremely) long overdue overhaul of my on top ten list, but that will be later.
No year-end review (especially of mashups) would be possible without mentioning the master himself, DJ Earworm. Once again he has taken the year's Billboard top40 list and mashed it.
This year a has seen a great many others try their hand at making year-end Mega mashups. The best of the the bunch was made by MashupGermany.
And just to round things out, here are a few of the better, more informed year-in-review mashup lists.