For temps, I remember a few basic C to F conversions and just estimate from there:
0=32
10=50
16=61
28=82
37=98.6!
Or, if you're good at remembering numbers:
0=32
5=41
10=50
15=59
20=68
25=77
30=86
35=95
40=104
Reminds me of the 9x tables - one digit increases while the other decreases.
