Gender bender was one of the things that I discovered thanks to the manga/anime genre and mostly, thanks to the, certain anime series that made me interesting in this topic. I found that there's almost no such series at west, so Japanese titles mostly here. But not only.
If you know any others worth watching/reading/playing, feel free to let me know.
Ranma 1/2
Yes, legendary Ranma 1/2, the story about "karate kid" Ranma Saotome that is cursed to change into cute girl every time he is splashed with cold water. Awsome, very humourous story with fantastic characters. I remeber I bought first volume of the manga accidentally at bookshop, barely knowing anything about it, but I fell in love instantly and have all the volumes published in my country on my bookcase.
Kampfer
This series has a chance to be Ranma of the modern days. It has everything - cool idea: a guy that turns into girl to participate i the street fightining game, nice girls around him, lot of humour. But it seems that authors forget about one thing - a plot. And it's visible, that it's just a harem series, without any other male characters (while Ranma 1/2 had a legion of cool male and female characters). Still, it's cool and worth watching.
X-Change
The series of the visual novel games (somehow like interactive, choose your own adventure ilustrated stories) about a guy that turns into girl (the same one in the first three games and the new one in the fourth). Much more erotic than the others here (it's +18 games), still funny and cute. What I liked the most was a big number of different final for each game - you could find the way to be guy again, become lesbian, end as a lover of the teachers or as a sex slave of some pervert and the others...
Shezow
I mentioned before, that I haven't seen much western series. But Shezow is something great. A superhero parody with gender bender twist. Protagonist finds a magic ring that his dead aunt used to turn into superhero named Shezow. He puts it on and finds that he can turn into superheroine as well. Very funny series with of of references.
Kokoro Connect
Something serious here. A story of the gang of friends (three girls and two guys) that are involved into series of body swapping accidents. Imagine that you're doing something and suddenly, you find yourself in the body of your friend. Interesting, heart throbbing story with lot of romance and drama. I like it much and have all the volumes of the manga at my bookcase. I hope someone will release novel here too, since it's longer and better that the manga.