Leslie tops the list.
Looks like there've always been a few girls named Leslie, but it went from predominantly masculine to predominantly feminine in the 1950s. Now it's almost exclusively a girls' name.
Kelly was about evenly split for boys and girls in the 1950s, but again, the girls took over after that.
In our preschool co-op, though, there's a mom named Kelly and a dad named Kelly.
People have always been naming boys Taylor, but around the 1970s it caught on big - and was an even bigger hit for girls.
Lynn: same story, but earlier.
A steady trickle of boy Ashleys was suddenly engulfed in a tidal wave of girl Ashleys in the seventies.
I had it in my head that Ashley was fairly common for men in bygone days - perhaps because of the references to Miss Emily's mythical beau Ashley Longworth in "The Waltons". But apparently it only ranked 693 in the 1890s.
"You can't call a boy Winnie" but you can call him Robin (with or without the Christopher).
You can call a girl Robin too, and from the 1940s on, lots of people did.
Ethel isn't exactly the hippest name these days, but up until about 1900 it was quite the rage for girls - and there were a few boy Ethels too.
The craze for naming girls Lindsay hit in the 1970s.
Dylan really caught on for boys in the 1980s, and brought in a few girls after the 90s. For some reason I expected to see more girl Dylans, but no, that one is still mainly boys' territory.
Despite ending in an A, the name Asa (after the King of Judah) doesn't seem to have ever been popular as a girls' name. It's pretty obscure as a boys' name too, but seems to be on its way back after falling off the chart completely in the mid-20th century. Asa was very big in the 19th century though. Every Stumptowner knows the name Asa Lovejoy, who founded Portland, Oregon, but missed out on the chance to name it Boston after losing that famous coin toss to Pettygrove.
ETA: And of course there's Shirley (see below). And Madison and Morgan, and Courtney and Whitney, and Lee and Dee. Kerry is about evenly split.