123.45 is a common one and its usually pretty funny
123.45 in the US is reserved and is not supposed to be used for aircraft-to-aircraft communications. Here is a quote of the FCC reg I pulled from a website:
FCC Rules, Part 87.303(a) reserves 123.45 for flight test operations with the following restriction: Mobile station operations on these frequencies are limited to an area within 320 km (200 mi) of an associated flight test land station.
I have read in many aviation groups, however, of pilots mistakenly thinking that this frequency can be used for aircraft-to-aircraft communications so if you are scanning it you may pick up activity.
These days, 122.75 over the US is the only authorized frequency for this activity, as per the US's FCC regulations, but again that does not mean you won't hear pilot chat on the other frequencies mentioned.