ANAPOD
YAMIPOD
SHAREPOD
MEDIA MONKEY
SONGBIRD

Personally, I use Windows Media Player (WMP11) but WMP won't support iPod's naturally (Microsoft vs Apple etc) so you need a third party plugin;

http://www.mgtek.com/dopisp/

Only problem is, the free trial download (fully functional) only works for 30days - so if you want to use it after that, you gotta buy a licence. Not expensive (think it worked out to about £8.50 or something) but seeing as iTunes and all the above are free.... Personally tho, I like to be able to use my iPod with my WMP as I use WMP's Library for all my music playing anyway, so makes sense (for me) really......