Ah I just thought of one I got about a year ago... They were selling Pilotwings Resort for the 3DS for $2, definitely felt like I got a lot more than $2 of entertainment out of it.
That's actually the most common "deals" I tend to find... there's a lot of quality DS and 3DS games being sold used for next to nothing. I occasionally pick a few from the $5 and under group and generally really enjoy at least one or two of them.