MAKR 2009-04-09 a list zipper. This was briefly discussed on the chat today by RS, kruzalex, and teo. The implementation could be like:
proc lzip {l1 l2} { set res {} foreach x $l1 y $l2 { lappend res $x $y } set res }
would lead to
$ lzip {a b c} {1 2 3} a 1 b 2 c 3
A more generic form is available as [interleave].