dc - 24 chars 25 28
dc is a good tool for this sequence:
?[d5*2+d2%*+2/pd1
dc -f collatz.dc
21
64
32
16
8
4
2
1
Also 24 chars using the formula from the Golfscript entry:
?[3*1+d2%5*1+/pd1
57 chars to meet the specs:
[Number: ]n?[Results: ]ndn[d5*2+d2%*+2/[ -> ]ndnd1
dc -f collatz-spec.dc
Number: 3
Results: 3 -> 10 -> 5 -> 16 -> 8 -> 4 -> 2 -> 1
