Swiff Player is a very small media player devoted to one thing: SWF files. It's a very tiny, 700kb standalone app that plays only SWFs. It's a really simple app, so there's not too much to say.
-Tiny: As I said, 700kb. Very tiny, and also....
-Portable: Yes, it is portable. There is no portable download on the site, but if you use Universal Extractor, you can get the EXE. Or you can install it, copy the files, then uninstall.
-Full Screen: You can now watch a SWF movie in full screen. Most excellent.
-Seek bar: It's a little weird, since in interactive SWFs, it jumps from frame to frame, but if you're watching, say, a flash movie, there is a seek bar.
That's about it. No downsides, its a good little app. However, if you get a media player that does alot of codecs, such as Media Player Classic or VLC media player, those will most likely play SWFs, which would make Swiff Player kinda redundant. But I have noticed that Swiff Player might open SWFs faster than MPC or VLC.
Visit Swiff Player website for download